Linux has a general measurement of server load called, creatively, 'load'. It's a numeric value that tells you how many processes are waiting to run at once. Elliquiy's server can handle a load of up to 4 without issue, because it has four drives and four cpu cores. So it can do four things at once, generally.
Swap is 'virtual ram' - it's disk space being used as memory, only roughly about a million times slower. That isn't an exaggeration, it really is a million times slower. This means when the virtual memory of something that is actively being used gets used, the OS starts swapping it in and out of memory, called thrashing. Which is what Elliquiy was doing the slowdowns, but, it shouldn't have been - there was a gig and a half of RAM left free. In Linux, you control how aggressively the system swaps out applications with the swappiness parameter. 0 means it won't swap until necessary (in theory), keeping applications in RAM, 100 means it will swap everything it can, saving ram for the disk cache.
Since Elliquiy isn't a fileserver, we want to keep applications in memory. And that wasn't happening, even at zero. v. frustrating.
Sphinx is the search engine for Elliquiy and Blue Moon. It makes an index of every single post on the site, and allows some pretty advanced features, though we only take advantage of a few of them.
* Vekseid is happy to explain things, though. : )