If it does almost nothing - may be not. Otherwise you'll be doing something in the main thread which will take time, unless you also squeeze concurrency (i.e. multitasking) into one thread and then again, why not use multiple threads already.

On a high enough resolution, especially with 5K-6K displays a single-threaded software-only compositor is absolutely going to have horrible performance. Even on Full HD it's actually quite noticeable