I've created a 16gb USB with Kali 2018.4.

Created persistence through the following method:
(for some reason the Kali Documentation didn't work for me)
https://devanswers.co/guide-kali-linux-2018-live-usb-persistence-windows/

mkdir -p /mnt/my_usb
mount /dev/sdb2 /mnt/my_usb
echo "/ union" > /mnt/my_usb/persistence.conf
umount /dev/sdb2

*swapping out sdb2 for my USB's ext4 partition name which was sde2

This method successfully created persistence, however immediately after booting into a freshly created usb
with persistence using this method, Kali becomes unusable and slow. e.g. Clicking firefox takes 30 seconds
to load. Moving windows causes freezing for 15+ seconds. Once firefox does load, even loading the google
homepage causes a freeze. Youtube locks the system up for minutes.

When I boot into a non persistent live usb of Kali 2018.4 it's super snappy and fast - it seems that something within the persistence creation causes the pc to run out of memory or suffer greatlyin how it manages the hardware.

my system usually runs things pretty well
core i5 7 or 8th gen, 6gb ram, gtx1070...