Solved zxfer subcalls of "zfs get props..." become slow in long run

Hello, I am using zxfer to replicate/backup ZFS snapshots (created by zfSnap) to the backup server.

zxfer calls this commands internally (read dataset properties):
zfs get -Ho property,value,source all zroot
zfs get -Hpo property,value,source all zusbbackup/backups/pf-server/zroot
...same calls for each source and target dataset...

Send/Reeceive is OK, but after month or two of server run, this zxfer commands become extremely slow (eg 10mins and more).
After restart, everything is much better.
Whats wrong, any ideas? Thanks!
I am running all zxfer replicating jobs sequentially now, so ZFS is less stressed. After few weeks it seems to be solved.
Also, increasing RAM helps a lot.
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