Uh yeah. I am not going to try to respond to whatever that statement is supposed to mean because it does not make any technical or logical sense whatsoever. The BPS standard htaccess files and code do NOT slow down a website at all and can actually speed up a website significantly if you are using the Speed Boost Bonus Custom Code, which is very similar to the “busy” htaccess code that WPSC and W3TC use – “busy” seriously???
I believe you are using SSL/HTTPS, which is known to slow down a website. That is common knowledge. If you have added additional custom code to BPS Standard htaccess files and code then that additional custom code might cause some slowness.
Ok now for some rational logical information: Activate Default Mode and benchmark test website performance. If there is any difference in speed then some additional code that you have added in your root htaccess file is actually causing some slowness. My money is on the support person is guessing, which happens quite a lot. They look at the BPS htaccess code and think to themselves “this looks very complex so it must be the cause of the problem”. Ugh.
Other things that commonly cause slowness: Caching plugins and Cloudflare. Yup seems odd, but these 2 things are the most common things that cause website slowness due to a variety of issues related to server configs or other random factors. So you need to eliminate these things if you are using these things.