We can't tell, we have no idea where it is slow - let alone why.
Start by profiling your app and finding out where it's taking time, and how much it's actually taking. Then you can start f]think about ways to improve it, and have a benchmark for how big an improvement you are getting. Manual profiling would involve adding Stopwatch class
] instances to your code, and logging the time various bits take, or you can use the Visual Studio profiler
] if your version of VS supports it.
Chances are that multithreading won't help significantly unless it's targeted at parallelable bottlenecks - it's not a magic bullet, it involves adding threads (which add processing time to support) each of which needs a free core to execute. Once all the cores in your machine are in use, thread get suspended waiting for a free one and do not run until one is available. Adding threads indiscriminately can slow applications significantly as each thread uses more memory and requires extra processing power to manage and switch the threads.