Regardless the MTP protocol with large numbers of files has always been a slow slow beast. Compared to ADB, MTB is a joke in transfering large numbers of files. I have never done clear testing but have seen transfers of 2hr vs 10 mins on the same group of photos.
What I have found is the more files the slower the pixel transfer. The less files at the same amount I'd data the faster it is.
10 gig worth of 5mb photos vs 1x10gb video file is hours vs just a few mins. Kind of the opposite performance you want from a smart phone as much of the data transfered would be multi photo audio files etc. Although now with the camera capability huge video files @4k are a reg thing.