|So if I'm reading your post correctly:|
- When connecting wirelessly over the VPN from one site to another, it's very slow
- When connecting via a wired connection over the VPN from one site to another, it's fast.
If this is correct, then it's pretty obvious the issue would be the wireless network. If the problem is your wireless, there may very well be nothing you can do about it. Wireless is subject to a lot of issue that slow it down compared to wired connections which is one of the many reasons I'm not a fan of wireless. It's insecure, it's subject to latency and lag issues that you don't get with wired, it's slower and well, you pay for the convenience of it by having to put up with all the aforementioned issues.
Including changing the BTU size, and various other twick-settings in the VPN connection, and Wireless NIC connection`s property settings.
I'm thinking you meant MTU as a "BTU" is a "british thermal unit" and is a measure of how much heat is produced by something like a heater.