Hello Ahmed,
Apologies for the issue that you are facing.
When we delete an attachment the corresponding file in the disk also needs to be deleted. So deleting attachments is relatively slower that deleting posts or users. The speed at which attachment can be deleted depends entirely on the specification of the server in which your WordPress site is hosted.
Can you give me an opportunity to find out what’s happening? Share me access to your WordPress install. Let me have a look and if I am not able to fix it then I will give you a full refund.