ok. so we assume device is ok.
Did you try test program and compared elapse times?
0.3 is not slow as such.
Could be interesting to see where you land.
What is myData ?
One big array ?
gives me 10.000 files on usb stick.
(unmount / mount before test to avoid cashing)
Elapsed time: 1.54938
So under 2 secs.
This is win 10. Qt 5.7.