I did that--or I had the host do that. Here's what the host said, though. I'll pass this along and ask whether you think there's validity to it because it deals mostly with xenforo:
In terms of MySQL, you have a couple issues. First off, your server is reaching the maximum connection limit. This is caused by code in your site where connections are opened to the MySQL server but never properly closed. This will cause your MySQL server to reject any further connections once it reaches this limit until slots open up. As a result, you'll see errors, slowness, etc.
The way to correct this is to fix your site code so that any time a connection to MySQL is opened, it is closed properly. The next thing I notice is that you have a lot of "JOIN" queries that take place without the use of indexing. This causes fragmentation, slowness and other issues with databases. It would be advisable, and best practice, to audit your site code and fix it so that any time a JOIN is performed, it is done with indexing. ....
The code fixes are definitely going to be the most helpful in terms of performance, so I would suggest implementing those as soon as possible. Please let me know if you have any further questions, and how you wish to proceed.
Are these "code fixes" something that xenforo can do?
You could disable "Enable Delayed Insert SQL Queries" - this causes long connections, though it should generally only be a couple. It's probably what he saw but didn't realize that they're actually internal to MySQL's implementation.
The join comment is random, especially without an example. (Obviously note that add-ons can add bad queries, bad joins, etc -- this is often where heavy queries appear from.)