Technically speaking, both are releasing builds at similar rates. PCSX2 has a complied build from the 31st. PCSX2 may have less giant leaps in performance, but it's also over a decade old and we're likely hitting performance saturation, likewise with Dolphin or emulators for older games (even though things like BSNES require high end processors for the most accurate of emulation; simply playing the games has hit saturation ages ago). PS3, because it's more recent, has awhile to go, and it's more obvious when big changes are made. You also have to keep in mind that this emulator really didn't pick up until 2017, and it has been in operation since 2011.
RPCS3 will seem like it's making massive jumps, but the performance jumps are for a few; eventually it'll plateau before they start making huge strides again. After all, less than half the games tested are considered playable, and even fewer of those are actually Peace Walker/P5 good. Plus, they haven't tested all games against all regions, and I doubt they've been tested against all patches and updates.
As far as I've read, they'd have to start from scratch with the PCSX2 coding since the back end is a mess of hacks and whatnot, which is also why they're slow on bigger jumps. If they have to scrap stuff and redo it in a neater fashion, that's going to suck up time, too.
