regarding clock speed Iām not aware of any restrictions on the gnrc_border_router example (though there are some on space, which most STM32 MCUs should fulfill). It might get slower and depending on the traffic it might get overwhelmed, but I think no one ever made any experiments for that specifically.
We have 6LoWPAN border routers running on Contiki OS and TI CC1310 @ 48 MHz ā RAM is the main issue, 20k is enough for up to ~100 nodes in the network. No heavy traffic, though ā except OTA fw updates, but we perform them one by one, not simultaneously.
I believe RIOT should have similar or lower hardware requirements, so a border router on STM32L4 should be fine.