If any steps cannot be run, try restarting into Safe Mode with Networking following How to enter Windows 10's Safe Mode.  You can also run the Repair commands via the Command Prompt at boot in the Advanced Startup Options Advanced Troubleshoot options which may work best from bootable media created from Media Creation Tool .

1) Turn On and run System Restore in Windows 10:  Make sure System Restore is always turned on for C drive and has plenty of disk space apportioned (5-15%) as this will be your first line of defense and allow you to roll back any undesired changes that affect performance. If you have a restore point before problems began, trying rolling back to it now.  If Windows won't start you can System Restore or do other Repairs from Advanced Startup Options including the booted media.

2) Type Update in Start Search box or browse Start>Settings (gear icon)>Updates and Security>Windows Update, select Check for Updates, expedite these through requested restart, then check again until no more are offered.   If you have Windows Update problems there is a new Windows Update repair & reset guided walk-though to resolve them: 

3) Once all Windows Updates are installed with no more available, right click Start button to open Device Manager, import any drivers still missing, or update or reinstall any in error with a ! mark, from the PC or Device maker's Support Downloads webpage. Compare drivers available for download with the ones presently install to make sure you have the latest.  It shows how to trace and find unknown device drivers here:  Updating a driver. - Microsoft Community

4) Type msconfig in Start Search box, right click System Configuration Utility to Run as Administrator.  In the tab for Services tick the box to Hide all MS Services, disable everything except anti-virus, any sync, stickies, printer with scan.  All others are freeloaders on System Resources, cause conflicts , spy on you and do not need to Start with Windows unless you know for sure you need them on stand-by at all times.  If in doubt you can Google the name or ask back here. 

Then move to the tab for Startups, which will take you to Task Manager to do the same thing.

Repeat this step for Startup folder by right clicking Start to access Run box, copy/paste shell:common startup press Enter to open Startup Folder.  There is also a new location in Version 1803 at Settings > Apps > Startup where these items can be toggled, so check there too.

5) In your browsers' Settings, check Add-Ons or Extensions to disable all but Flash and any Reader helper: How To Disable Plugins/Add-Ons/Extensions.  Change the Search to your desired one:  How to Add Any Search Engine to Your Web Browser.  I would have several browsers on your task bar so there's always one that's best for the task.  I use Firefox, Edge and Chrome in that order of preference.  It's easy to share bookmarks between all of them in their settings under Import Favorites from another Browser.  Set the preferred one as Default in Settings > Apps > Default Apps. 

6) Remove any bundled third party security utilities and use the built in and exceptionally secure and fast Windows Defender Security Center. It includes easy to use features such as Antvirus, Firewall, Network security, Smart Screen Filter. It also includes the ability to block ransomware and other types of malicious activities - and its FREE! No need to buy a subscription.

7) Download, install, decline trial version, update,  then run full scans with Malwarebytes, the best free on-demand scanner.  Afterwards you can turn off the Malwarebytes Premium Trial in it's Account Settings.  There's no need to buy the real-time protection in the Trial unless you get repeatedly infected using only Defender. 

8) For problems with apps  Troubleshoot problems with an app - Windows Help .  Many apps can be Reset in Settings > Apps & Features by choosing the App's Advanced Settings.  You can also Re-register Apps in Windows 8 and 10.

9) Type CP in Task Bar Search box, open Control Panel, in Search box type Maintenance,  choose Security and Maintenance, choose Maintenance, then Start Maintenance.  You can make sure Maintenance is set to run regularly in the Maintenance Settings.  This is all the maintenance a good install of Windows 10 needs.  Third party optimizing and tweaking suites can ruin Windows.

10) Type CMD in Task Bar Search, right click Command Prompt to Run As Administrator (or boot the  Windows 10 Installation Media to access Repair My Computer on second screen, then Troubleshoot Options, open a Command Prompt) Copy and Paste the following commands each followed by Enter key:

sfc /scannow  

If damaged files are reported run up to 3 separate times to try to repair files. 

If irreparable files remain then the image file they are compared against may need repair.  To determine this type Powershell in Search box on taskbar, choose Run as Administrator, copy and paste or type precisely these commands followed by Enter key:

DISM /Online /Cleanup-Image /CheckHealth

If Component Store corruption is detected, this will tell you if it is irreparable or can be repaired:

DISM /Online /Cleanup-Image /ScanHealth

If the image is reparable, run:

Dism /Online /Cleanup-Image /RestoreHealth

It may take some fiddling to get these correct.  Sometimes inserted media won't be read, but mounting the ISO file itself (by double-clicking) to use as source will. Proofread closely, watch spacing, ask back if you need help or consult this guide to DISM:   DISM - Repair Windows 10 Image - Windows 10 Forums

Once the image is repaired then the broken System files should repair by running again:

Sfc /scannow

11) If these still won't repair after running 3 times and performance problems persist I would do a Windows 10 Repair Upgrade which is a simple procedure that resolves many problems by reinstalling the OS while keeping your files and apps - although it may resolve problems better if you choose to save nothing, after backing up your files first. 

12) Problems with the Start Menu and Task Bar can be fixed by [How To] Re-register Start Menu In Windows 10.  Many other troubleshooters are at Settings > Update & Security > Troubleshoot.  Type Control Panel in Start Search, choose Icon View, then Troubleshooting, at top left View All to see many more.

13) Run a full Disk Check, if necessary from the Command Prompt.  For problems with Hard Drive or SSD run the maker's Hard Drive Testing Tools.  If none for your HD brand then run SeaTools for Windows or boot SeaTools from a USB Drive.   Check for newer SSD firmware.  Type Disk in Start Search to Defragment & Optimize Hard Drives & SSD's.

14) Since they make it easy, those with Dell or HP PC's should run the built-in Hardware diagnostics by powering up the PC while pressing the Dell F12 or HP ESC or F2 keys.  Test your RAM - Test with Memtest86+ overnight.

15) Compare the present BIOS version from Start>Windows Admin Tools>System Info to latest UEFI/BIOS firmware Update on the PC or motherboard model's Support Downloads webpage, follow the instructions carefully (sometimes in the download's Read Me file) to flash the BIOS update.  For PC's within one-year support warranty, ask for guided help from manufacturer's Support. 

16) Analyze repeat Errors in the Event Viewer as explained here:  How to Troubleshoot Problems in Windows Via Event Viewer.   You can zero in on processes (including Startup processes) that are causing problems following How to use Process Explorer, Microsoft's free, supercharged Task Manager alternative 

17) Another thing to try with persistent problems is to follow How to use a local account to fix issues on Windows 10 to create a new Admin account to see if the issues are present when you test running from it.  If not you may have a corrupt User account, which may or may not be reparable using the repairs here.  As shown in tutorial above, test that all programs run from the new account to reinstall any that don't, copy your files over, then when you're sure nothing is missing you can Delete User Account in Windows 10.  At any time you can also convert a Local to a Microsoft account or vice versa:  Switch to Microsoft Account in Windows 10 & Switch back to a local account from a Microsoft account.

It may be necessary to enable the built-in hidden Administrator used only in emergencies to create the new account as shown here and here.  Afterwards be sure to hide the account again.

18) If necessary you can also use whichever method works here to access Advanced Startup Options - including from booted media - to attempt a Startup Repair to check over the boot files, System Restore, Go back to old Build, Reset Your Windows 10 PC or access the Command Prompt to run any of the commands given. 

19) If serious problems persist you can open the Windows 10 Installation Media for your version on the desktop to perform a repair upgrade using the Windows 10 ISO file, also a good way to apply the latest Windows 10 software version which will always be available in Media Creation Tool.   This will resolve most problems, but if not you can also run the automated Reset Your Windows 10 PC to reinstall the OS but you'll need to reinstall your personal programs afterwards.  Reset may also reinstall corrosive Factory Bloatware.

20) An automated tool is also available starting in Windows 10  to Start fresh with a clean installation of Windows which will remove Factory Bloatware when the Reset does not because it has been customized by PC maker. 

21) What we're seeing especially in Windows 10 is that everything depends on the quality of the install so if you want to invest in the very best install of all do this gold-standard Clean Install Windows 10 which clears the hard drive to get it cleanest..  You will get and keep the best possible install to the exact extent you stick with the steps, tools and methods in the link.  I will assist you in the Comments section there.

22) If problems persist please start a new thread with a descriptive title in Installation forum, describe problems in detail including any pictures using Attach Image button in reply box or screenshots following how-to Screenshot using Snipping Tool.  You can even Record steps to reproduce a problem - Windows Help.  We will try to help you resolve any remaining problems until you have a perfectly running install. 

If you'd like to ask a question about these steps or ask for my help in your thread, ask or provide a link in Comments below.