What is a decent speed for a card that can be used as internal storage in marshmallow?
My SanDisc Ultra was according to Marshmallow not fast enough ;( - but worked. I couldnt find anything on the Definition of the Speed for Marshmallow to swallow those SDs without an notification.
But I assume that all are not fast enough and you will get notifications nonetheless:
Here are Classifications of Speed:
For Samsung Flagship (S6 e.g.) 
UFC 2.0 = 100mb/s
emmc    = 90mb/s (most of the phones have this)
SD Cards:
UHS3 30mb/s
UHS1 10mb /s
class 10 10mb/s
class 6 6mb/s
(those Speeds (obviously) are depending on your phones Controller)

I feel like that as Long as the SD Card Speed is smaller as the built in Speeds - your will get those error/info notifications. 
But I could get it to run with class 10 (no Problems) - Streaming movies / Music was no Problem 

Do you want to use the internal mode? (on what phone/purpose?)