The blackmagic debug probe https://gith.im/blacksphere/blackmagic is a JTAG/SWD
debugger that provides GDB server running on a microcontroller, talking to the
debug device via JTAG/SWD and exposing the GDB server via a CDC ACM modem
implementation. Blackmagic hosted is a PC program that talks to debugger via an
additional remote protocol implemented on microcontroller, using the same
CDCACM implementation. E.g reading from the device is done in bigger chunks,
not single words. Running on KOTD (exactly  5.13.0-rc4-1.g1c805bd-default) read
speed dropped about 1/3:
 Reading flash from 0x08000000 for 1048576 bytes to /tmp/bla
 Read/Verify succeeded for 1048576 bytes,   52.020 kiB/s
while with 5.3 it is above 85 kiB/s  or even about 100 kiB/s on a newer USB
chipset.

strace brings down the speed substantial, so I do not think that is useful for
debugging the problem.

Hints and help welcome.