it's understood that (at least) on GSM Treo 650, if a custom ROM is used, there is a major db cache loss.

The issue is discussed in detail at Shadowmite's forum, at following thread:

It seems that with normal ROM there is around 11MB db cache space. But with a custom rom this is cut into nearly half of it having (5500-6000KB) db cache.

I think it should be considered an important issue. That's why I wanted to inform everyone about this...