performance – Local WP install giving often ERR_EMPTY_RESPONSE error

WordPress works most of time, but slow.
But very often I get ERR_EMPTY_RESPONSE, I think the reason it’s just because it’s slow.
If I refresh the page, sometimes it works.

It’s a local install of an online site that doesn’t have any problem.
My machine is a 16GB MacOS Catalina with Apache 2.4 and PHP 7.0

I don’t think it’s MySQL problem. I see no slow queries.
The site has woocommerce and totalcache installed among other plugins.

Don’t know how to troubleshoot this…