Also having performance issues and SpringBoard errors with simulators. I've reinstalled Xcode 15 beta 5 after removing Developer folder content in system and user libraries to try to workaround those issues, but that didn't help. Besides what has been said in this thread, I did noticed iOS SDK is not properly registered in Xcode ever. SDK version remains 21A5291f and is not updated with 21A5291g for the latest SDK. I'm attaching a screenshot with that particular issue.

Xcode 15 Beta 6 seems to improve some of these issues. Previews and simulators are still slow but I have not seen springboard crashes in this Xcode release so far.

To me, simulator performance improved so much after removing all content on /Library/Developer and /Users//Library/Developer folders. It seems like there were remaining files on those folders interfering between the latest xcode beta version. Currently, I see many of the following simulator log errors, but don't seem to cause major performance issues. 21:41:37.870260+0200 apsd <APSUserCourier 0x10e506160 development 0> invalid presence without server timestamp. Rolling token... 21:41:37.977540+0200 familycircled Connection for <APSConnection: 0x600003b0c2a0> differs from current queue!!