Or it's the latency chasm between RAM and disk. A cursory googling to refresh my own memory, SSD is at least 100x slower than peeking at RAM and possibly much

Or it's the latency chasm between RAM and disk. A cursory googling to refresh my own memory, SSD is at least 100x slower than peeking at RAM and possibly much more, depending on who's numbers you trust. It seems at least plausible to save time by compressing pages in memory rather than going to disk. Certainly for low end or consumer systems not blessed with gobs of RAM.