Recent-ish (10.7 and later) versions of macOS tend to fall apart under memory pressure with traditional hard disks-- more so than previous versions did or than Windows does on similar hardware. My hunch is that they changed something in their virtual memory subsystem to takes advantage of characteristics of SSDs that isn't quite so friendly when your drive has significant seek times.

A consequence of this is that an SSD upgrade in machines of that era can be quite dramatic, especially if you're memory-constrained. Might not be too helpful in the 2008-era machine (an SSD might be bus-limited there, not sure what generation of SATA those machines have), but the impact on a 2012-era machine (if it shipped with a traditional hard disk) can be huge.