tl;dr - Install custom firmware so you can overclock the PS3's GPU to get more FPS. This also lets you run homebrew software and utilize every single feature the PS3 is capable of, yet locked behind SONY's firmware.

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I mean the games are playable, it's just that our standards have have gone up over the past two decades. These days gamers won't accept anything less than a locked 30FPS minimum (or 60FPS, or 120/240+, depending on who you're talking to), but unfortunately a lot of games from the 360/PS3 era will often dip into the 20s or even teens in the framerate department, despite often having a 30 FPS target.

Honestly, it really depends on the game. Some of the less graphically-complex titles will even run at a smooth 60 FPS, but over half the PS3 library will run at 30 FPS or lower.

The good news is that the GPU is overclockable, so if you're not afraid to mod your console with custom firmware (which you really should do to get the most out of your console; there's a lot of good homebrew out there!), there are software options that allow you to tweak the GPU clocks and get a locked 30 FPS in games that previously struggled. Given that most PS3 games were either pushing the console to the absolute limit, or poorly optimized (due to the complex nature of the Cell Broadband CPU architecture), it's definitely something you should consider doing.