This is not a problem of PIO Core or even GDB. This issue is linked with on-board CMSIS-DAP interface. I have a few other boards based on CMSIS-DAP and they work very fast. Would be good if you contact with ARM mbed engineer and ask why this board is slow for debugging. Maybe, they will propose new bootloader/firmware.
P.S: Thanks a lot for the feedback. I’ll release NXP LPC development platform with a new mbed v5.4.5.