As tested on 20.12.2, Kdenlive starts background rendering jobs for generating proxies at the default priority (0). This can cause noticeable UI latency in the main window, even though it's a background task and the user should be able to work while it's happening.

My suggestion is to run these ffmpeg or other background tasks with niceness 5 and the equivalent on Windows. Or, if desired, make the niceness configurable in Kdenlive settings.