I've been haunted by this issue for last 6 months, and today I just somehow randomly found the solution out.
Just simply revert to a previous version of an IME at the Windows 11 settings. That's it.
https://support.microsoft.com/en-us/windows/revert-to-a-previous-version-of-an-ime-input-method-editor-adcc9caa-17cb-44d8-b46e-f5b473b4dd77

It's so hidden and buried under many options so it'll take some time to find it. If you find the correct option page, disable any fancy options like expanded kanji or something like that, and simply turn on the 'Previous version of an IME' under Compatibility.

I believe this fix actually solves many keyboard related stuttering and issues on many older games, not just Space Marine.
Now go replay the campaign and let us see in Space Marine 2. Peace!