Is this on device or desktop and is it an optimised compiled application or in the IDE? Out of interest (because I might like to play with it!) which Task Manager are you using to alter the priority?
Basic4ppc applications are single threaded and the Sprite library runs on that single thread so whatever the thread is it has nothing to do with the application unless you are running in the desktop IDE where, although I don't know the architecture, there are several threads running under the IDE.
I believe the device IDE is single threaded and an application running under the IDE runs on that thread which is one of the reasons there is no debugging capability in the device IDE.
This is on the device in the IDE.
It's something i've just noticed so i'm not sure what situations it will effect a sprite.
Using FdcSoft TaskManager I can see 3 threads running at normal priority when starting the IDE.
After loading the project and running the code it is still 3 at normal.
At some point one thread goes to a high priority, don't know why it changes but it does effect the speed of sprites.
Running an optimized compiled app not using sprites I can also see 3 threads.
So this does appear to be a b4p thread not specific to the Sprite library.
I'll have to complie and see if it still does the same.