Installing an SSL certificate doesn’t slow down page loading. You may be worried that using SSL will slow down your site and slow down the loading of its pages. Fortunately, encryption has no significant effect on the speed of your website. In most cases HTTPS refers to HTTP/2, which is actually a standard of the HTTP protocol. This protocol is designed to reduce page load time by 50% by compressing data and reducing related processes. What you need to know here is that the WEB has been using HTTP since 1991 and then upgraded to HTTP/2 to improve performance.
To prove that installing an SSL certificate does not affect the speed of your site, you can check the speed of sites such as Facebook that has an SSL certificate and pay attention to their speed. Of course, speed can be affected sometimes, but it is very small because we are talking about millisecond delays. Sometimes this delay is due to server distance, which you can not normally help with.