It's a little bit different for everyone from what I'm reading. Some people get a message saying their card is slow and might slow performance and some don't. I didn't and used a 128 gb from san disk.
Initially it didn't work at all. I downloaded a bunch of giant games to test it and all saved to internal storage which was only 16 gb (my mistake) and I had to go into settings to get it to work. I had to (migrate) from sd card to internal a few times until it finally worked.
And then it worked. Really worked. Any app or game I downloaded saved to sd card. Apps that did not probably move before I could move manually. And best of all downloading movies offline from vudu finally worked and saved to SD card. The fatal flaw is google play music doesn't recognize it so it just saves offline to internal. Fine if you have more then a 16 GB base storage or if you don't use music a lot. I never noticed any slowdown in mine. I've had several apps running, bluetooth on, hotspot on (always) and even my moga connected.
That being said if you just need music or pictures and don't need the apps to be moved your better off just leaving it as portable storage. The only apps that can't be moved are system apps which is unfortunately google play music
. Hoping google fixes this soon.