long, large rsync slows down over time ?

[Ensel Sharon]

I am moving a large number of files (about 40 GB, comprising almost 3.5
million files) from one machine to another using rsync.

I am noticing that the longer this rsync process runs, the slower it seems
to go, and the more CPU time it is taking on the destination (FreeBSD
6.1).

Is there a reason for this ?  Is there some aspect of rsync (or
FreeBSD) that just starts to bog down after transferring 1-2 million
files?

When the job started, load on the destination was between .2 and .8, and
now, four days later, it is up to 4.0+  ...  and as I say, although I have
not measured, throughput does seem to be slowing down.

Any thoughts / comments ?