C# Parallel coding on existing working project (has parallel operation but it runs slowly).

The parallel operation works fine for 1 thread and on up to 4 threads, where 4 files process at better than 20% longer than one thread. As the number of threads increase the speed decreases. By 8 threads (running 8 files) it slows down to about twice the runtime of 1 thread. The software should run on all available threads.
There are a lot more C# projects to come.