My SD card was working fine for a few weeks, but yesterday I get slow write speed and it shuts off.

This is using the reccommended Samsung, retail store, not a fake card. How to fix this issue? I do not feel like buying another memory card.

Hello, there. Sorry for the inconvenience. Would you please take a photo of the SD card and try to use another SD card for test only?  

Try one from your drone(s)

Even I faced this. Been using the same card for some time & suddenly one day the camera kept saying slow card even at the lowest possible resolution. It was fine after a few hours. Seems like a bug to me.
The latest firmware was released after I faced this & I haven't faced it again so far. Fingers crossed.

I had this error once too. Reformatting the SDXC card fixed it immediately.