rapidly panning the camera slows down the program?

i have a doubt, that rapidly panning the camera can reduce framerate. (in my case, from 30 to 26 at worst)
i thought it was only the setText calls of onscreenText that slows down the program, but now i have changed to faster text updating method, updating a dozen of text strings doesn’t affect framerate, nearly. but if i pan (or move) the camera quick enough, my framerate can drop, while the framerate is stable at 30 or 29 if the camera moves slowly.
i couldn’t find other source of performance hit. so does Panda do some kind of cache on the camera ?

Does it happen only the first time you pan a camera into a particular directory, or every time?

not only the first time, not every time either. sometimes i have to move the mouse like crazy to see the drop of FPS meter, sometimes it easily happens.
i saw in the PStats, the red part(draw) had more chance of rising when i pan the camera.
i’m using Panda1.81, DX8 renderer.

today i test it again, and it is not there anymore.
probably it was caused by graphic card overheating, as the weather here is much colder today.