I don't know UFS, and I don't know the filesystem internals well enough to give an educated answer.

However, I experienced with all filesystems I tried a slowdown after some time.

Therefore I try to put directories with heavy read/write access into separated partitions/filesystems.

At my Linux (Gentoo) workstation there's a directory with >150000 files and around 43MB, which is updated daily. (It contains the software package information).

In order to get some more performance, I'm using a virtual filesystem kept in the memory, which is loaded and saved into a single file at the boot/shutdown process.

In your case, if performance is important, I could also think of two partitions, which are regulary copied/formatted.
Something like:
(dev/part1 mounted at /mnt/mountpoint)

mkfs /dev/part2 
mount /dev/part2 /mnt/tmp
cp -a /mnt/mountpoint /mnt/tmp
umount /mnt/tmp
mount /dev/part2 /mnt/mountpoint
umount /dev/part1
[download]

That's, however, just a suggestion to start from and possibly not practicable for you, I'm really very interested how you/others solve this problem.