File transfer is slow

When writing an 8MB image to a board I see this:

u-boot-rockchip.bin
      8,896,712 100%    1.03GB/s    0:00:00 (xfr#1, to-chk=0/1)

sent 346,038 bytes  received 35 bytes  692,146.00 bytes/sec
total size is 8,896,712  speedup is 25.71

8774144 bytes (8.8 MB, 8.4 MiB) copied, 10 s, 877 kB/s
17376+1 records in
17376+1 records out
8896712 bytes (8.9 MB, 8.5 MiB) copied, 10.1574 s, 876 kB/s

The last bit seems to be the 'dd'.

If I drop the 'oflag=direct' from USBStorageDriver.write_image I get:

4198912 bytes (4.2 MB, 4.0 MiB) copied, 1 s, 4.2 MB/s
17376+1 records in
17376+1 records out
8896712 bytes (8.9 MB, 8.5 MiB) copied, 2.82611 s, 3.1 MB/s

which is a bit better. Why is direct I/O needed?

commit 27087817f168fbfe5594dd4e2603e336abc05834
Author: Jan Luebbe <jlu@pengutronix.de>
Date:   Thu Jun 18 09:13:27 2020 +0200

    driver/usbstoragedriver: use dd with oflag=direct
    
    This avoid write-caching in the kernel's page cache, reducing disruption
    of concurrent processes and making the progress information more useful.
    
    Remove the leftover debug message of the current working directory.
    
    Signed-off-by: Jan Luebbe <jlu@pengutronix.de>

 labgrid/driver/usbstoragedriver.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

This doesn't make a lot of sense to me. Why not let the kernel handle the caching?

Read labgrid/driver/usbstoragedriver.py and commit 27087817f168fbfe5594dd4e2603e336abc05834, then compare the reported transfer behavior with and without dd's oflag=direct. Done means establishing whether direct I/O is required for this driver and documenting the rationale or defining a justified change.