Finally converted my last "home" partition to ext4. The time it takes to tar it all up (350 GiB) has dropped from 3.5 hours to 70 minutes; a full fsck, from 2.5 minutes to 10 seconds. Quite happy.
(I was a bit surprised, as these are mostly big files; I expected ext4 to have less of an impact here than with a partition full of little files, scattered all around. Shows how little I know about this stuff.)