Well, Windows is not a real time operating system which could guarantee processing and response times.
So your observation should be considered "normal behaviour".
Did you try to raise the process and/or thread priorities to ensure that your application is served best?
Note: the ANT DLLs create threads as well, in order to handle communication with the ANT USB stick. So you might have to tweak that code which is provided as source code as well.
Cheers,
OMB