The default driver for No.12
Front Panel Type C USB 3.1 Gen2 Header (USB31_TC_2)
is slow.
I have the Asmedia ASM-1x4x/2x4x/3x4x/3x4x/107x drivers version 1.16.54.1 WHQL
installed. It doubles the speed at the front panel and is approximately as fast
as on the back of the Intel USB 3.1 Gen 2 port.

Asmedia ASM-1x4x/2x4x/3x4x/107x Drivers Version 1.16.54.1 WHQL
Link:
Asmedia ASM-1x4x/2x4x/3x4x/107x Drivers Version 1.16.54.1 WHQL

PS.
It is not on the driver DVD
and also not on the Asrock Z390 Taichi Driver page.