iPad Is it bad to only have 8gb of space left on my iPad Air 2 128GB?

absolutely not!

 

My iPad mini 4 16 GB has only 700 MB left

 

It's only a problem if you happen to need 9GB of space.

 

Have you noticed any slow-down?

 

Linux and windows SSDs have something called trim that lengthens the live of your ssd and stops performance decrease.

http://www.buildcomputers.net/trim-support.html

Also if you are formatting your own you usually keep a few GB blank

I would imagine iOS does all this naturally

 

Think about it this way, if only having 8GB free was detrimental how would that impact all the 16GB phones and iPads. I wouldn't let it get under 1GB but otherwise You should be fine.

 

i have a 16gb Air 2 with 4 gb left and its fine. My iPad is a consumption device so I can get away with it.

 

I have a 16gb Air 2 and I frequently have less than 1gb free and have never noticed a slow down.