What is a decent speed for a card that can be used as internal storage in marshmallow? Marshmallow allows sd cards to be used as internal storage.

I would like to know what is the benchmark for the cards that can be used seamlessly with Marshmallow as internal storage. 

It would be good if someone can the technical details of the cards to look at before buying cards for 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?)