...I remember this bug being present in all games of the third series...

There was no such bug in the third series, I can't say anything about 3.5 - I don't know, but in the first release of the fourth game, the bug was very severe until it was patched to version 1.3. The game started normally, but after some time, severe slowdowns appeared - the memory filled up, and it was impossible to play without restarting. This did not depend on the machine's power, the amount of memory, its current automatic cleaning, or the game settings. In theory, the time until the slowdowns began could only depend on the size of the map and the complexity of the game. :(