Hey, just chiming in to say I'm having the same issue all of a sudden (I've had my board for around half a year now and I believe this issue started a day or two ago).
Uploads are fine for an instant-- it says it's hovering around 3 - 5Mb/s-- but then immediately crashes down to around 60 Kib/s.
Uploading a 3 MB file took long enough for me to google other peoples' concerns in this forum, find no solutions, and then register an account and start typing up this reply before the file finished uploading.
I've already tried reformatting my SD card with 64k clusters but no success; same behavior as before.
Running M122 command immediately after upload gives the following data for my SD card:
"SD card 0 detected, interface speed: 20.0MBytes/sec
SD card longest block write time: 184.6ms"
Any thoughts/suggestions?