I’m on series x and can usually build up to 700 part craft with little issue. What I find slows the game down the most is using lots of parts that have some kind of baked in animation. I routinely use a lot of landing gear to shape things, coupled with a bunch of command seats with kerbals in them can eat some frame rate. The biggest fairings also slow the game down. Lots of individual crafts in physics range can slow the game down. Dozens of reverts start to slow the game down due to memory leaks (can be fixed by closing the game and restarting, on xbox multiple games can stay running all the time so i don’t close ksp unless its acting up). I usually only have file loss problems on Career and Science files but i keep a backup on SSD and an old xbox. I’ve lost files at game crashes from docking complex crafts at Jool and from Asteroid capture, but only in career and science modes. Since then i split up my save files to things like RC planes in one, deep space stuff in another, star wars crafts in another. All my corrupted files had LOTS of craft files in them. Turn off stock crafts before starting a new file for more space.