Very slow performance under Yosemite on Mac mini caused by systemstatsd
The culprit was indeed systemstatsd. The way I understand it, that process both produces a number of system stats, dumps them into a file which it later analyzes. For some reason, that file never got reset, resulting in a giant stats file that took serious resources to analyze.

What I did was stop the analysis daemons,

sudo launchctl stop com.apple.systemstatsd
sudo launchctl stop com.apple.systemstatsd.analysis


remove the stats dump file (which was about 3 gigs in size at the time)

cd /private/var/db/systemstats/
sudo rm snapshots.db


relaunch the daemons

sudo launchctl start com.apple.systemstatsd
sudo launchctl start com.apple.systemstatsd.analysis


I never figured out why that dump file never got cleared, but suspect it'd something to do with the PRAM, so for good measure I finally reset it by rebooting and pressing CMD, ALT, P, R when the boot chime was heard until it was heard a second time.

The end result is a mac that is very much snappier than before. Here's hoping that zapping the PRAM did eliminate the root cause.

Further reading: What is the use of snapshot.db?