Base PS4 had SATA2, which means the SSD would be between 2-3 times faster than a mechanical HDD. But it's not only that, sustained transfer speed is just one of the advantages of SSD's, but the other that clearly goes to the top is the IOPS, which are the operations per second, and that parameter is, to put it simple, something that can have a huge impact in loading performance. A regular HDD has around 200 IOPS, depending on it's rotation speed (RPM), a mediocre SSD nowadays can have easily 50-60K, which is almost 300 times more. It is true that Sony nor the game devs created PS4 games to take full advantage of SSD's, but even so, games load much faster. In many games the loading times are almost halved, and in the worst cases you can have an improvement of 30-40% faster loading.
Even on PS3, the difference isn't marginal, for the last reason I've told you, the IOPS. I have tested also HDD and SSD in my PS3 Slim and Super Slim and there are games that can improve as little as 10-20%, but others can improve up to 40-50%, but not many. Anyways, even putting an SSD to a PS2 has improvements, not about loading games, but about loading assets such as game covers or other info that needs to be loaded as fast as possible. It also helps the PS2 to get cooler and reliability skyrockets as there are no moving parts such as the ones within HDD's.
