Any quality SSD is going to be significantly faster than a hard drive. The best 7200rpm drives might be able to manage 180MB/s streaming reads, while the best 5400rpm drives might be able to do 120MB/s on a good day. Though the Seagate ones Sony uses get maybe 70-80MB/s. Decent SSDs these days will get about 520MB/s sequential reads, and do better in random 4K reads. So games should in theory load up to five times faster.
The question is whether it's worth it. The minimum drive the PS4 OS requires is 160GB. Though given the size of the games, you'd want at least 512GB. Cheapest one I found on Newegg here was $380. Almost as much as the console itself.
The HGST Travelstar 7K1000 seems to be the fastest 7200rpm 2.5'' drive, getting about 120MB/s. But I'm not sure if it works. Weird how PS4s are being finicky with replacement drives.