I'm sorry but Windows 10 Mobile is just slower on most phones that came with 8.1 preinstalled.
You can try to do a factory reset, but that wouldn't make much of a difference. If you'll like to have 8.1 back, you can download the Windows Device Recovery Tool.
Download and install the tool. Connect your phone using a USB cable, open it and wait for it to detect your phone. Click your phone and follow the directions in the tool.