I'm using a sandisk adapter, with 32 gb micro sd sandisk ultra class 10, on usb 2.0 speeds (at least it always used to transfer at 2.0 speed). It is in NTFS format. I usually transfer video files between 50mb to as much as 1.5 gb in file size per file.
I've transferred with no problems for years doing this but suddenly it has just become slow. I don't know exactly what speed it's transferring at but for example, it used to take me only a minute or 2 to transfer like 4-5gb off the card but now it takes me 7-10 minutes. Sometimes it'll say 2 minutes remaining it looks to be going at a good rate but then suddenly the reading tells me it will take 8 minutes now after 20-25% transferred.
It's the same card and same card reader I've been using so it was transferring before fine, just don't know why it just suddenly now transfers slowly.