On average it seems a half decent SSD can save 20 seconds on loading times across the board.
It's obviously nowhere near as fast as the SSD and I/O interface in the PS5, which is also hardly the point.
The real comparison (as per the thread title) is to the standard mechanical drive in any PS4, where the difference would not only soon add up during extended play, but also allow for quicker access to dip in and out of.
Granted this applies moreso for the Pro (SATA 3 600Mb/s vs. SATA 2 300 Mb/s internal) or as others have stated with a USB3 external drive in general (500 Mb/s minimum).
A good 2.5" SSD will be in the 400 - 500 Mb/s range for continuous reads, four times faster than a very good HDD. However, random read speeds are only up to 0.5Mb/s on a HDD, versus 20 - 40 or so with an SSD - which is its real advantage, especially as a system drive.
So the lower peak bandwidth capability isn't always a problem when using SATA 2 and the SSD still delivers an improvement in general.