Welcome to the forum.
Can you launch Activity Monitor and see what is going on? If you can get AM running, look at the CPU stats and click on the header of the column "% CPU" until the big hitters are at the top. It may be that Mail is still working on getting the file attached, which could be bogging things down.
Also, right click on Macintosh HD then Get Info and look at how much space is left on the drive. If it's getting full, that can slow down your Mac.
You haven't given us information about your Mac, so next time, let us know what Mac (from About this Mac), how much memory, how big the drive is and how full it is, plus the version of macOS you are running.
And what have you tried? Have you rebooted and it's still running slow?