I have been experiencing the same on a general level for some time.
It feels like the mouse movement buffer is being dumped/dropped/overloaded when frame rates are low, so your sensitivity temporarily feels very low.
It is evident in a large map with a decent-sized server on TF2:
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If you face the wall away from the main map area and move your mouse, you get 'correct' movement according to your sensitivity settings. Framerate 80-100+
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If you turn to face the action and move your mouse, your sensitivity feels effectively 10-20x less, according to your actual framerate drop vs the 'baseline' of 80-100fps.
The problem is that the game is still at a very playable framerate (40-50), but the mouse is momentarily much, much less sensitive. When you just get into the heat of battle, your mouse will suddenly behave unpredictably -- and your twitch movements will be molasses.
This leads me to believe that the problem is not sensitivity .. but with mouse movement information being permanently lost, so the sensitivity algorithm has an incorrect measure of time between movement. If we could pinpoint that loss of time between movement, an adjustment algorithm could be created that would make the mouse more 'jumpy' in low frame rate scenarios, but decidedly more accurate according to the user's hand movement.
Jason Fisher
jason.fisher@gmail.com