Q:

What does setting Thread.Priority = Lowest really mean?

In an effort to speed up the startup of my resource-hungry app, I've moved various startup tasks to background threads and marked those thread with 'Thread.Priority = Lowest`.
However, those low priority threads still execute pretty much in parallel with the application (as it loads its UI), as evidenced by the timeline on the ANTS Profiler.  My understanding was that Lowest meant that the CPU will handle all higher priority threads first, then get the lower priority threads.
Is my understanding flawed?

A:

The threads may be scheduled with the lowest priority, but they don't wait at the back of the line. They will probably still get enough CPU time slices to gobble up certain resources that are the real bottlenecks, like hard drive access. It really all depends on exactly what you are doing. 
Is the initialization computation-intensive? Or web intensive/hard drive intensive. A multi-threading approach is going to be most effective when different tasks use different resources, or to allow computationally intensive operations run without blocking other operations.
A single-threaded approach could feasibly order the tasks to make the application appear to load faster, where-as the multithreaded approach may mean that everyone gets their hands in at the same time, possibly even getting in eachother's way.

A:

Lowering the priority doesn't mean that the thread will always be the last one picked to get a time slot. If the lower priority thread hasn't got a time slot for a while, it will be more likely to get one. That way lower priority threads will run slower, but not completely stop.
Also, if the main thread is waiting for something, like for example waiting for the disk drive to return data, the other threads can run in that void. If the main thread does a lot of disk I/O, there will be a lot of holes to run other threads in.
If the CPU has more than a single core, the load will be more evenly distributed between threads. No matter how high priority a thread has, it will still only run on one core.

