From a UX perspective, I can see doing simple syntax highlighting on the UI thread...so long as it is something with small, bounded execution time. I don't quite get why completions and other stuff lags the UI thread, as it seems obvious that looking that information up is expensive. I can't tell if that is what's happening, or there's something more going on, such as coordinating the communication between UI/worker threads becomes costly.

I've seen it in a bunch of IDEs though, especially those in managed languages. You're typing, it goes to show a completion, and then....you wait.