That definitely seems like a fair improvement considering dimensions. Depending on that speedtree pop though, it could be expanding in memory and caching to disk. That size in a pop is huge. I can imagine a small 1k area of just couple thousand adding up very quickly. It's hard to gauge popsize in RAM during render and things can jump. On my old old Xeon with only 4gb memory, i had to use a spare HDD for my "memory" and because it was always caching to disk, vegetated renders were painfully slow. They worked, and completed with a CTD because of no RAM, but sloow.
Windows too, automatically allocates 1.5 installed ram to virtual system memory, so if you have a lot of RAM, you have lots of virtual disk space as well, and windows will manage it accordingly to keep physical RAM when handling large files.