There is a known issue with memory leaks in Roon Server that’s currently getting addressed, and which exhibits exactly in the behavior you described, as is being extensively discussed here:
Perhaps you want to upload your database as requested by Roon Labs in the linked post (and subsequent posts)
There’s no reason for a significant performance difference between ROCK and DietPi, both being rather minimal Linux-based variants. Most likely, ROCK might be a tiny bit faster as it’s probably running even less processes, but it’s probably insignificant.
Of course, faster hardware always helps, but the issue is most likely in Roon Server (either the one described above or some other one) and this would be the same with ROCK and another Linux variant, as they all run the same Roon Server, and faster hardware may not be a permanent solution until Roon Labs fixes the issue.
There’s no real reason for DietPi to be less reliable either. It would give you more insight into memory behavior (if you are suffering from the leak) than ROCK, which gives you nothing.