I finally found the solution. The problem si in corrupt OptiX cache DB file. The OptiX kernels are cached in a file to avoid that taking a long time: On Windows in %LOCALAPPDATA%\NVIDIA\OptixCache, on Linux in /var/tmp/OptixCache_$USER. If you delete the corrupt DB file named cache7.db, the OptiX renderer will make new cache7.db file and the problem with lagging will disappear. After first load of render kernels into the cache, switching to the rendered view, enabling OptiX viewport denoising or rendering with OptiX works fast again. More info about this problem can be found in these threads: https://developer.blender.org/T73076 https://developer.blender.org/T73076