Can you try joining a multiplayer game on a remote server, ideally on a small-ish map with no or very few other players, and check what framerate you get?
Also see if the simulation seems to be playing catch-up in some way, e.g. appears to run normally (or slightly slow) and then suddenly run several frames at once.
In multiplayer mode the game should lock on to the server's rate and run that regardless of the timing otherwise used in local play. For that reason you will often see 32.9 fps on Windows in local play, but 34 fps on the same Windows machine joined a server running on Linux.
Also, try running a dedicated server on your own machine (run openttd.exe with the `-D´ commandline switch) and check what the rate it with the fps command. If the rate then is correct, try joining the server and see if that affects the rate.