Slowness problem Error 500

Hello to all the team.

When you are on this page (for example) and click on the bottom page 3.

The theme takes a long time to load page 3.

In some cases, error messages appear like this one for example: 504 Gateway Time-out nginx

On other pages, this message also appears:500 Internal Server Error
An error occurred while processing this request.

The wordpress technicians told me that the problem came from the theme.

Maybe due to plugins? Which plugins are bad for the theme? I will delete them myself.

Thanks,

Have a good day