Hello,
You are using the second design type of the off-canvas block that uses Ajax requests to show the content http://prntscr.com/pjgkdi The speed of the request-answer depends on your server. So, either edit the mobile menu and choose the side menu design (1-st or 3-d) for the off-canvas content https://prnt.sc/pjggut instead of the second one (1-st and 3-d types do not use Ajax to display menu) or contact your hosting provider to speed up the server response time.
Also, read the post on how to speed up the site https://xstore.helpscoutdocs.com/article/100-how-to-speed-up-my-website
Regards