It's where temporary files are stored....turn tablet off, then press and hold volume up and power key....use volume button to scroll down to wipe cache partition, power button to confirm. It will then as to reboot, confirm with power button
I've noticed the lag too since the update. Happened on my Tab s6 too. Its one of the reasons I upgraded to the s7+. Well... that and the attachable keyboard. Kind of annoyed but also relieved that it's not just me.
I find pressing the button that brings up all the open apps then going back into the app I want seems to un-clog whatever is going on. And I'm not someone who has every app I have installed on this thing open at the same time.
There's a bug somewhere that Samsung has to work out.