Recognized this problem first with LRC version 9.3 on Window 10 and hopefully did update to 9.4, but there's still the same problem. Every time I press "Play" or "Preview" in the slide show module, LR is preparing all the pictures one by one and slow as usual as it is since LR 5.
It's like it doesn't cache the previews anymore. Cache is activated and may use up to 10 GB of space.
As it turns out, this is just happening with files in TIFF format. Slide shows from raw files in RAF oder CR2 format are cached and processing happens just once.
Actually they haven't - that thread indicates that it's 'working as now intended', which has completely destroyed any ability to use the slideshow module. It takes minutes to render 200 images on a powerful machine, followed by a spinning beachball. In the past, it would have started in seconds. This is a deal-breaking bug and needs to be fixed. 9.2.1 was the last time the module was usable, so I've downgraded to it again - but that eliminates all the performance fixes for the new machines, as well as new features. Very frustrating