ZFS zfs vs ufs for 4t and 5 usb drives

I have some usb drives external.

I have zfs on 2 of them and ufs on 3rd.

I run deluge and surf net with firefox.

I see zfs churning around the arc and fighting python for disk writing.

Firefox sometimes slows down.

Would I be better off removing zfs and just using ufs with softupdates?

I see slight amount liek 300M swapped.

16g ram 6 core amd.