It’s a little better than your read. The ~400x speedup is in comparison to using ZRAM, which, in very brief terms, compresses memory by creating an in-memory compressed block device and assigning that as your swap space. So your “swap” is actually a compressed chunk of RAM, not on disk.
This was significantly slower than normal memory access because page faulting when looking up something in memory then fetching from swap was, itself, expensive. Regardless of how fast that swap was. When your swap is on a storage device that overhead is comparatively tiny, but when it’s just another chunk of memory suddenly it’s what you’re spending most of your time on.