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.