Very slow performance under Yosemite on Mac mini

I've got a Mac mini, model late 2009, that's after some time on Yosemite begin to perform very poorly. Safari takes about a minute to launch occasionally, as an example.

Granted, I've got a number of background tasks running (Dropbox, BitTorrent Sync, AirServer), but that's unchanged compared to before upgrading to Yosemite.

My main suspect is the very high network usage. The process kernel_task is using a lot of LAN bandwidth, occasionally downloading 60 MB (that's megabytes)/s for a few seconds before going down to zero, for no apparent reason. The total amount of bandwidth used has exceeded 14 terabytes (!) after a couple of weeks of uptime.

I've got a NAS that I use for sharing files and backups, but since the process using the bandwidth is kernel_task, I don't know what to think.