I used zram + swap for years. I dedicated 25% of my memory to zram. The problem is that zram would get filled with infrequently used data, and disk swap would get the frequently used data. Once that happens everything slows down.
Zswap tries to fix that be creating a compressed swap buffer in memory. Older/less used data will get written to disk, but fresh/frequently used data will stay in the compressed ram buffer. That’s my understanding, at least. I don’t remember how to query Zswap usage stats.