From the perspective of a computer engineer SSDs are painfully slow. Waiting for data on disk is slow enough that it is typically done by asking the OS for the data and having the OS schedule another process onto the CPU while it waits. RAM is also slow although not nearly as slow. Ideally you want your data in the L1 cache which is fast enough to minimally stall the CPU. The L2 and L3 caches are slower but larger and more likely to have the data you want. If the caches are empty and you have to read RAM your CPU will either do a lot of speculative execution or more likely stall.

Speculative execution on CPUs is a desperate attempt to deal with the fact that all memory access is slow by just continuing through the code as if you know what is in memory. If the speculative execution is wrong a lot of work gets thrown out (hopefully nothing unsound happens) and the delay is more noticable.

Bluntly an SSD only system would probably be an order of magnitude slower. I'm also not sure switching to a new process (or even thread) to load from SSD would be viable without RAM as it would likely invalidate a lot of cache triggering more loads.