For sure, 9 will improve your performance. Also, you will get extra widgets which are not available in 8 versions. For that, you could have gone with custom logic with java script. Replacing this with the widgets will improve performance.
Mendix 9 does have the performance-bot that does give you pointers on possible improvements, so that helps. Apart from that, I have not experienced drastic changes nore any mention of it in the release notes.
For a customer I'm using both LTS versions 8.18.22 and 9.18.3. Both projects are around the same size on disk and in the modeller. Running several tests and even converting the 8 project to 9, in my experience Mendix 9 runs a lot slower and is more user unfriendly. Compiling seems to take longer, progress bars and the completion info messages occassionally don't show up, the tabbing between windows doesn't work very well and the caching like you said is sub optimal to name a few..
Mendix 9 does provide some neat new features, but in my opinion you're better off sticking with 8 at the moment.