The free version is not limited in any way at all regarding connection speed and Linux is generally always in the best shape out of all the clients as that gets developed first and is in constant use.
Usually I'd say if you were getting handshake errors that you might not be forwarding UDP/51280, but you can connect a Windows client, so that can't be the case.
Regarding a slow connection speed, I've seen one case where this happened before and tweaking the MTU had some effect.
Please raise a support ticket here: https://support.logonbox.com/app/ui/wizards/support
and send over the log files from both your clients and I'll take a look at those for you.
The file I'm interested in is service-app.log and should be in the client install directory under logs.
Chris.