People look at posts like yours and get it through their head that if they use Python, their website will crawl. Nothing could be farther from the truth.
If your website is anything like the majority of websites, the server-side code will spend far, far more time waiting for IO than doing any sort of processing. The language choice is almost irrelevant. It's essentially just glue to join HTTP queries and database queries.
OTOH, if you do need to do some actual processing, you might be better off just doing that in a compiled language (C, Rust, etc.) and using the web framework glue to hook it up to the web.
TL;DR: Using C++ for web server-side because "it's fast" is premature optimization. Don't prematurely optimize.