Not a bug.
With multi-threading introduced in 1.3 you have greater performance, but the game now can no longer disconnect the action from what's being displayed.
It used to be if you pressed x4 acceleration that graphics could update once and the game data would update for times. Basically what happened was it replaced some of the graphic processing for game data processing.
But with multi-threading, though it's faster, it's too complex to juggle this data separate from each other and the game can only update game data at the same rate as the graphics. So if you press x4 it could only do four times the game data if it can also do four times the graphics. Clearly, this is a problem for most PCs, who can just about run the game 1x. Those will not see any acceleration.
I think it's possible to turn off multi-threading somewhere. This should allow you to accelerate your game, but might reduce overall performance.
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