pruning file takes time too long

[hendranata]

helo
i am using: Version:TrueNAS-12.0-U8
our pool using 8tb x 4 (RAIDZ2)

cpu usage is relatively low.

memory ram 16gb with usage :
free 0,5gb
zfs cache 9.4gb
service 5,8gb

we have a lot of backup file there.. the problem is when pruning the folder which is contain many files there.. the process of deletion / pruning takes too long..
do u know why? i suspect that due to low free memory ram.. what do u think?

thanks