Proper way to process the huge amount of data from a sensor using Parallel.For
Usually it makes absolutely no sense to use a parallel loop with a lock inside it. The performance loss you are talking about comes most probably from it. They cannot run in parallel because they have to wait for each other. The lock also costs some time and if it happens to often, the parallel loop might actually be slower then the normal one.

You might gain some performance if you redesign it in a way that does not requrie synchronization.