Super slow file transfer via OTG and system lag Android 9
Probably the USB OTG adapter.

on some controllers I have to flip the USB around, one side of the USB C runs at USB 2.0 speeds.

The other side will run at 3.0 speeds.

If your tablet has a full USB port - then im baffled. running fast at start then slowing down is usually a sign of write caching. sometimes it doesnt measure the speeds right.