Gmail API - loading takes more time with 50 threads - C# ASP .NET WEB API

Threading is not a Magic Bullet that will unconditionally increase your app speed: A thread needs a free core in order to run, so if you spin up X threads and there are X / 2 cores in the machine, you cannot run all thread simultaneously, and at least half of them will be blocked waiting for a free core. How many exactly depends on the load on the rest of the system: all the other threads are also competing for free cores. And since threading adds overhead (both memory and processing) generating too many threads can actually make your code run slower than it did as a single threaded task - particularly if you use any external processing, which is implied by your code.

I'd start by running it as a single thread task, and using the Stopwatch class to find out exactly where your bottleneck is: then concentrate on that area to find performance improvements. Just bouncing straight to multithreading won't necessarily improve your performance at all, depending on what your Request.Execute is actually doing, and what resources it requires.