FiveM uses GTAs files. So even if you have FiveM on your SSD, it still needs to load all the data (world, vehicles etc) from your HDD with GTA on it.
FiveM itself is pretty small. Less than 1GB on my SSD (not counting the cache that can be much higher depending on the servers you play on!).
So if you can manage to fit GTA on your SSD you can probably squeeze FiveM on there as well