For me it lags too, and I wasn't even playing on my wooden PC, so I can't imagine it's my PC that can't handle it...
As for the mission itself: it's a funny thing you did with the stones 'falling off' the mountain, but I'm not sure if I like it. I didn't play it with sound enabled, but did you put any sound effects there? If not, I think you should consider it. Also, I think it would be better to give the player a little time to process what happened. Now when I start the mission, I'm being attacked right away and I see all these stones moving around which is very distracting. You could make it so that the rogues have to walk down before they start to fire, and maybe send the troops in a little later. Once the stones have fallen down the player receives a message like 'Oh no! We are trapped!' (or whatever) and right then all the troops come in... Of course, all of this is up to you, I'm just thinking about how I would make this mission.
And I don't really know the story behind it anyway.
The map has some weird stuff. I don't really like the waterfalls except the one in the center of the map (that's a nice one). And although the original maps had it too (mission 20 TSK if I remember correctly), I absolutely hate 'left out tiles' in suspended bridges. The exit of the cave looks a bit sloppy too, although I admit that making stuff like that look nice is not always easy. Overall your mountains look pretty good. Interesting mission, good job.
Edit: I tried your new .dat file Grayter, but it made no difference. I don't see how lag could be caused by the .dat file anyway - I think it must be in the .script file.
Edit 2: However, deleting the .script file doesn't change anything, so scrap that.