My guess is that you were swapping. In addition to slowness, this explains how switching to an external hard drive for the files you're compiling would help, si

My guess is that you were swapping. In addition to slowness, this explains how switching to an external hard drive for the files you're compiling would help, since it would reduce contention on a single hard drive. Next time it happens, check if you're out of memory and using a lot of swap space/virtual memory. If so, you just need more RAM. Also, the best upgrade for any computer these days is an SSD, an HDD being the slowest part of modern computers.