--> The GRE Tunnel connects to our datacentre and users are experiencing slow wifi speeds
It is unclear what is causing the slow speed, and if the users in the data centre or the users in the branch office are experiencing the problem. If a user connecting with an Ethernet cable has no problem, then the issue is likely the WiFi.
On a GRE/IPSec tunnel, changing the MTU size often increases the throughput. Try the values below:
ip mtu 1400
ip tcp adjust-mss 1360