I have already posted a feedback item about this issue in the Feedback app but thought I'd open a thread here to add some more information. I am running Windows 10 build 10162 x64. When the "Hey Cortana" feature is enabled, TeXnicCenter hangs for about 15-30 seconds when building a project.
I investigated a bit and found out that there is a thread in SpeechRuntime.exe that does not seem to run its message loop appropriately, which leads to the DdeConnect function hanging in DDE clients. (TeXnicCenter can be configured to control Adobe Reader via DDE). DdeConnect internally broadcasts WM_DDE_INITIATE via SendMessageTimeout to all top-level windows. For the windows created by one thread in SpeechRuntime.exe this call timeouts.
In case it helps, here is a snippet to reproduce.
Would be great if this bug gets fixed. DDE is pretty old now but there are still applications using it and making them hang is not good.