Hi all,
I installed El Capitan two days ago on our 2010 Macbook, which has just 2 GB of RAM. Had all sorts of issues with the initial startup - it wouldn't boot, and I had to boot up in Safe Mode and follow instructions for deleting all sorts of cached files (found those instructions in another thread somewhere on this site). In any case, I finally got it to boot up, but it was running really slowly, not because of high CPU usage but because of high Memory Pressure, even when only running the Finder. It was weird, because none of the applications in Activity Monitor seemed to be using a ton of memory, but the memory pressure was yellow and we got lots of spinning beach balls and hangups (and other wierd stuff happening).
I followed all of the various suggestions listed here (disabled Spotlight, reset NVRAM, reset SMC, deleted Mail log files, rebuilt Mail, reduced transparency, etc) but what really seemed to work was: time. After about 20-30 hours, the memory pressure suddenly went from yellow to green, and the laptop seems to be working just fine.
I wouldn't say it's screaming fast (I was hoping the upgrade would make the system run faster than it did on Yosemite, as the reviews I read seemed to promise - this is, sadly, not the case). But at least it's working smoothly now, and as long as we don't open Mail (which we don't use anyway) it seems to stay green.
I suspect, with others, that the upgrade takes up a lot of memory because it's rebuilding something? Or indexing? Whatever it's doing, it's well hidden (and disabling Spotlight doesn't seem to stop it). We did not have FileVault enabled so encryption was NOT the issue in our case.
I haven't tried restarting it yet, so it remains to be seen whether the slowness will return after a restart. Fingers crossed.