Showdialog() is taking so much time in C# windows forms application

ShowDialog is a modal operation: it does not return until the form is closed.
So depending on what the form you called ShowDialog on is doing when it loads and whether it requires the user to close it, your calling code may wait some considerable time.

Since we have no idea what the form does, how it does it, or even what it looks like we can't c=give you specific instructions, but you could either use Show instead of ShowDialog (which return pretty much immediately while the form loads asynchronously); speed up the processing in the Form.Load and Form.Shown event handlers; or remove user input and close the form programmatically.

Do note that if you use Show instead of ShowDialog, you will need to monitor the form via it's Form.Closed event in order to "know" when it is finished if you want to prevent multiple instances.