[CentOS] OpenVPN problem

I'm having problems communicating with a remote server, with openvpn. I can link with ssh ok, but the link stalls when viewing any but the smallest texts. I tried reducing the tun0 MTU, though only from 1500 to 1400, but this seemed to have no effect.

Before trying a larger reduction, I'd like to clarify two points.

Firstly, is it necessary to re-boot the remote machine for the change in MTU to come into effect? Or is it sufficient to re-start the openvpn service? (I'm reluctant to re-boot the remote machine unless it is essential.)

Secondly, must the MTU be changed at both ends?

Any info on these two points gratefully received.