As you probably know, the percent of secure HTTPS web sites is growing every day. Moreover, even if you do not care about security, the latest news from Google suggests that you will have to move to HTTPS in any case, because otherwise you will see your site dropping down in search results.
Depending on the implementation of your site, switching it to secure connection may take from few minutes to few weeks. You may face functional problems, broken links and thigs like that. Imagine that after you have done everything required, you finally see your site working under the perfect green line of the browser address bar. The question is: what have happened with the site performance, and should you load test it again?