I'd like to report that latest F25 Server on RPi 3B is slooooow too.
It came with 4.8 kernel known for some issues on ARM, but after dnf update (it took ages
to complete) 4.10.13-200.fc25.armv7hl is no better.
I've found that anything that's writing to mmcblk0, gets blocked on maximum write
speed of 64 kb/s. Main guilty is... systemd, perhaps updating journal.
I've been playing with /sys/block/mmcblk0/queue/scheduler to change scheduler from cfq
to something else but no spectacular results.
The card is Class 10 Sandisk which performed very well under latest Raspbian ( where
latest means kernel 3.something ).
I have tested IO performance on budget Sandisk Criuser USB 2.0 stick (it acts as my squid
chache directory) and performance is roughly the same as on regular PC, so it seems that
the problem is inside the mmc driver.
BTW: Let me join your kudos for those bringing Fedora to RPi !