You’re probably correct, although I have no experience with zram so can’t be sure. But you’re absolutely right that PostgreSQL depends heavily on the OS disk cache for optimal performance. Lowering the PostgreSQL setting like Blaster M suggests won’t improve performance much, since all that setting does is tell PostgreSQL’s algorithms how much memory is likely to be allocated to the OS disk cache. Of course it’s best if it’s accurate, so you’re best off seeing how much memory is actually allocated to disk cache under heavy use before setting it, but it shouldn’t massively reduce performance if you don’t get it right.
Worse performance, not everything works, and depending on the country you live in and which VPN provider you pick a VPN can actually be a downgrade in privacy since a second commercial entity now has the ability to look at all your traffic and distil valuable data from it to sell. The better VPN providers say they don’t do this (and some probably don’t) but a lot of them will definitely do so.