Poudriere became very slow

[Philip Homburg]

I've been running poudriere for a couple of years. When I first started
using it, it was very fast. But now it is painfully slow.

Most of that seems to be related to pkg. 

Hardware: i7-4770K, 32GB memory, spinning disk, UFS.

Over time, it seems that poudriere spends more and more time in the
*-depends stages. The main thing that seems to be running is pkg-static.

It feels like pkg is constantly flushing changes to disk. 
There is also an issue with some packages in actually generating the actual
pkg.

Is there any magic setting that I missed? 

For historical reasons the system uses UFS instead of ZFS. I could rebuild the
system with ZFS if that solves the problem. But I'd rather not.