I have a SanDisk 64 GB Extreme U3 card, which is capable of up to 40 Mb/s write speed.|
First, I used the SD as is, out of the box, in the camera without any extra operations with it. The camera recorded like 20 mins of Full HD video. It seemed fine and I tried to record a longer video, however the camera started saying SLOW CARD, writing down videos of 2min max length, then waiting and starting to record a new 2 min video.
Then I formatted the card to FAT 32 with a special tool for Windows, and formatted the card again with the camera (it became exFAT). Since then it started to record videos fine.
So the advice would be, whatever size of the card (up to 128 GB), format it to FAT32 file system and see how it performs.