One of my sites is extremely slow. Module conflict?

Hello everyone,
I hope you are well.

The site of one of my clients is extremely slow. I tried to deactivate each module to see where it came from but nothing helps. GTmetrix doesn’t give me too much help.
Would you please where it came from?

I specify that it is a multi site. A B2B site and a B2C site on the same hosting with multiwoocommerce.

If I add the QueryMonitor module to debug then I get a 500 error.

I hope one of you can help me. Thank you and wish you a nice day!

Woocommerce : Version 4.0.1
Wordpress : 5.4
Theme : Avada with Child Theme

The page I need help with: [log in to see the link]