Thank you for your detail info.
Any WiFi related connection will be better via 5GHz, including GL router connects to the primary router/AP through repeater and wireless client connects to GL router.
For WAN: Wired WAN > Repeater 5GHz > Repeater 2.4GHz (Cellular modem and Tethering are not confirmed, as it is related to the cellular network and base station)
For LAN: Wired LAN > 5GHz WLAN (WIFI) > 2.4GHz WLAN (WIFI)
So, in all connections, use wired or 5GHz Wi-Fi as much as possible. It will gain a better speed experience.
Regarding the speed reduction after enabling VPN client, it may be these following reasons, please refer to: