I thought people would find this article on game performance using HDs versus SSDs interesting:
http://www.hardocp.com/article/2013/12/10/hdd_vs_ssd_real_world_gaming_performance
Not too surprisingly, they found that hard drives didn't affect framerates in a single game, including BF4 mutliplayer.
At the same time, they mentioned, but did not quantify, the benefit of SSDs on load times. I agree with what others have said in this thread: loading BF4 off of a hard drive is so slow that you can't help but think it has crashed while loading. Running off of an SSD is tolerable, but still much slower than BF3. Perhaps DICE will tweak that a bit in future updates.
I recall there was an issue with load times associated with running AMD cards back when BF3 was introduced. I wonder if anything like that is going on here (I'm on Nvidia this time around, but was using AMD then).