We have a tool, that produces "dotx" files. The tool can automatically add content based on some variables. The added content is inserted via the OpenXML "AltChunk" mechanic.
This worked absolutely fine in Office 2013/2016/2019, but with Office 365 we are seeing performance issues.
We even could reproduce it with a very simple dotx template: We just inserted 200 "AltChunks" with the content "ABC".
Office 365 Word needs more than 4 minutes to open this file. On a machine with Office 2019 this takes 5 seconds. It is kind of stuck in this "phase":
The content itself is "simple":
To make this even worse: If you have multiple Office languages installed, the startup time will decrease even more.
Via ProcMon we noticed that Office 365 scans the registry on startup and does a weird lookup for languages:
I already contacted the MS Word support, but he heard "AltChunks" and told me, that this is a developer related topic.
My theory:
Each AltChunk somehow triggeres Word to scan the registry and this takes time. That means: If you add another "AltChunk" or add another language the startup performance will decrease drastically. I'm not sure, why this happens now - Office 2019 doesn't show this problem (at least in our environment).
If you are interested I could send the test file to someone.