Posting in case anyone else has the same problem.
Since recently reinstalling M&B and Warband, I've found the same problem in both. Frame rates in menus and on the world map would be fine for a brief period, but then drop to such low frame rates that the menu would fail to notice clicks sometimes. GPU/CPU load was really low, and the game was fine in combat.
It turned out to be an issue with Nvidia drivers. Updating to the latest version resolved the issue.
Since recently reinstalling M&B and Warband, I've found the same problem in both. Frame rates in menus and on the world map would be fine for a brief period, but then drop to such low frame rates that the menu would fail to notice clicks sometimes. GPU/CPU load was really low, and the game was fine in combat.
It turned out to be an issue with Nvidia drivers. Updating to the latest version resolved the issue.