WP struggles editing/updating/sticking posts

From some months now my websites struggle navigating over the dashboard, specially when editing or updating/sticking posts. Sometimes even for posting comments. Taking 10-30 seconds to do so.

(*) Sometimes WP keep loading an edit/update when the post is already edited/updated live (but WP will keep saying: “waiting for saberespractico.com…” as it was still updating).

I also have troubles sticking posts to the front page and undoing so. It takes me sometimes 4 attempts to stick/undo them (and in some cases this leads to 503 errors).

I think it’s a database problem, but we can’t find it.

WHAT WE’VE DONE SO FAR:

Server wise everything is fine according to the company that manage it (php and mysql are updated, we have all the recommended modules and the server is more than enough for my overall traffic).

Query Monitor only shows 1 data base error related to Wordfence (I’m missing a table: performance_schema.session_variables). Wordfence says it’s a server problem and my server company say it’s better how is it right now than create that table and risk having deprecated errors according to MySQL documentation: https://dev.mysql.com/doc/refman/5.7/en/performance-schema-variable-table-migration.html

Anyway, WP health doesn’t show any additional problem and I don’t know what else can it be.

Any idea of what can be happening?

Thanks in advance!

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