I agree with the SWAMP's idea (some relationship between Poser and QT).

I found out the behavior which suggests existence of the active relation between Poser and QuickTime.

QuickTime cannot be uninstalled while Poser5 is running. (The message that there is a program which is using QT is displayed) . By terminating of Poser5, I was able to uninstall QuickTime.

Then,,,, I installed QuickTime, with the state maintained where Poser5 is running.
Then, the performance was maintained good until Poser5 was rebooted.
The performance got worse again after rebooting Poser5 (not a OS).

This behavior is suggesting that Poser5 try to use a QuickTime module actively itself if QuickTime was installed, I think.
The cause of this problem is not kinds of DLL-overwriting but a behavior of Poser5, I think.