On windows I don't have any problem at all on Reddit server 2

I wonder if it's network-related?

In a game I was playing with some friends of mine I was experiencing the same issues.
The problem turned out to be the server not having enough memory available for the game. It was swapping and killing the performances.
It might be a similar problem in your case, perhaps.

Server performance defining simulation speed is nothing new and totally expected but I doubt that's the case for Reddit servers. Also it seems that for some reason server is affecting rendering performance of clients in some cases which shouldn't really happen:

https://www.reddit.com/r/openttd/comments/hg9pbt/can_something_be_done_to_avoid_poor_performance/
https://www.reddit.com/r/openttd/comments/hhf5jj/bad_performance_when_moving_around_on_a_10241024/

When a network game is catching up with the server, it goes into a "fast" loop where drawing isn't given any time to happen. The best thing I can think off, looking at the info you give, is that this is happening:

• The server is clearly rather busy, and a game-loop takes your machine 28ms to handle. This is really close to what it needs to keep up (30ms), so it is not unthinkable that in burst your machine doesn't have enough time to run the game-loop
• Your client gets behind, some frames stack up to be executed
• Your clients wants to catch-up. It does this by running all the missed frames as fast as possible, completely blocking your screen
• You experience this as low FPS in bursts

This is far from ideal of course, but the problem is hard to solve. When the game-loop takes close to 30ms, all kind of weird stuff starts to happen on multiplayer games. I am not sure that there is a correct solution. I do how-ever want to prevent this "blocking" of the interface a bit more, and maybe show a message, how-ever short, that this happened. I dunno yet ..

But, more importantly, do you recognize this scenario? Could this be your case, you think? Or am I wildly off?

Closed based on IRC discussion.

When the client can't maintain normal speed in a single-player game, the game-loop and drawing simply run a bit slower; the UI may become slightly less responsive but there's no step-change in responsiveness and the FPS is generally consistent even if low.

In multiplayer, the game-loop can't be allowed to drop behind the server else the client would desync. As @TrueBrain described, the client catches up by skipping drawing on some ticks and running only the simulation: https://github.com/OpenTTD/OpenTTD/blob/master/src/network/network.cpp#L997

This causes irregular groups of dropped frames rather than the smooth performance degradation seen in single-player. In the worst case the client can barely keep up with only the simulation without being desynced, so drawing may happen very intermittently or even not at all for some periods. That appears to be the case here.

Usability could be improved by showing a notice when drawing is skipped, or even by disconnecting from the server if drawing has to be skipped for many frames in some time period. The game probably can't be made playable in this case.

In other words: Your computer is too slow to play a game of that size in multiplayer.

That's the very short explanation, yes.

I filed a bug because the effects of being "too slow" in multiplayer are completely different and much more user-hostile than in single-player [for the reasons above], and that was unexpected to me.