internet speed decrease after connecting VPN

Have look if the F5 VPN is using TCP or UDP for its tunneling protocol. This is a common problem with TCP over TCP connections when working with unstable network connections such as Wifi. Basically what happens is the wifi connection looses a packet (5-25% packet drop is quite likely over wifi), after which both the tunnel TCP session, as well as the inner TCP session are trying to recover their connection, causing a bit of a snowball effect if during the recovery process another packet gets lost.

The solution is to enable DTLS on the F5 VPN connection; https://support.f5.com/csp/article/K54955814

This enables the outer tunnel to use UDP instead of TCP, meaning that if a packet gets lost, only one TCP session needs to recover its session, which doesn't cause the snowball effect.

I've seen this myself a few times where people are working over unstable connections but not notice it until they connect to a VPN and the speed dropped to 10% of their normal speed - or less. Switching to DTLS brought the speed back to roughly 90% of its normal speed.