Hopefully this helps someone else out there, but I too was experiencing "Slow Card" and "SD Card Error" while trying to record anything greater than 1080P (I was mostly recording 2.7K so I could use 60FPS and image stabilizer)|
The SD card I was using is Lexar 1000x microSDXC 128GB UHS-II/U3 (LSDMI128CBNL1000R) (https://www.amazon.com/gp/product/B00U77V5KU/ref=oh_aui_detailpage_o02_s00?ie=UTF8&psc=1). What solved my issues was doing a Full Format, not just the quick format, of the SD card to exFAT with 128kb allocation size. After that, and based on recommendations from YI Support, I upgraded to firmware 1.4.0 (https://www.facebook.com/groups/XiaomiYiCam/search/?query=1.4.0). After I performed both of those things, the card has worked flawlessly and I have recorded video on 2.7K at 60FPS continually until the battery died (prior to the Full Format and when I was using firmware 1.3.3 I was only able to record 4-6 minutes before I got "SD Card Error" or "Slow Card").
I also purchased SanDisk Extreme 64GB microSDXC UHS-I Card V30 (SDSQXVF-064G-GN6MA) (https://www.amazon.com/gp/product/B01HU3Q6S4/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1) and after performing a Full Format to exFAT, the card has worked flawlessly with my camera on firmware 1.4.0
So those that are having issues I would strongly suggest that you perform the Full Format (yes it's painful to wait the ~1hr or so, but it fixed my issue) and upgrade to the new firmware 1.4.0 https://www.facebook.com/groups/XiaomiYiCam/search/?query=1.4.0
As for those that aren't using a recommended card by Yi (at the bottom of page https://www.yitechnology.com/actioncamera/specs/id/5), you are rolling the dice, but maybe my success might help you get your card to work.