Web browsers he difficulty with perfect framepacing. This is not important for demo-only tests but important for scientific tests, so reboot both OS and browser, before scientific critical TestUFO tests like pursuit camera. At the absolute minimum, restart browser. More stutters will appear the longer the browser runs due to JaScript automated “GC” events.
Make sure no system tray apps consuming CPU and run TestUFO as only app and only browser tab, don’t he Task Manager running while TestUFO is running, and quit all system tray apps.