will it eventually lead to slowing down the load time of the website
Adding thousands of rules to your
.htaccess file is not a good idea. If the
.htaccess file gets too big, then it could impact the performance of the webserver. I routinely see
.htaccess files with hundreds of lines and they perform just fine. You would have to add lots of these rules to get to the point where there would be performance problems.
does it affect any SEO or ranking
Google will likely never know that these redirects are in place as they are meant just for users that are told about incorrect URLs. As such, there is almost no chance that they will affect how your site ranks either positively or negatively. If somebody does link to one of these URLs that redirects, then Googlebot could find that URL and crawl it. In such a case, having the redirect is better than not having it because Googlebot would otherwise encounter a 404 error and not pass authority to your website from the link.
Besides adding redirects to your .htaccess, you could think about enabling MultiViews on your Apache server. With MultiViews,
/this_page.html can also be accessed as
/this_page which might eliminate the need for many of your redirects. Unfortunately, MultiViews creates duplicate content by serving the same page at different URLs instead of issuing redirects. From an SEO standpoint, you'd want to make sure you have rel canonical meta tags on all your pages before you started using MultiViews.