Poor website performance [duplicate]

It's hard to say what the exact problem is with out providing some type of query examples, but I would first suggest having a DBA review your common queries to make sure that you're not missing any indexes. Firing off a bunch of full table scans on a large dataset can cause significant performance issues.

My second suggestion is to review some type of solution for caching. Specifically, full page cache. Something like Varnish will allow you to cache the entire rendered page so you don't have to execute duplicate, expensive queries. https://varnish-cache.org/ - remember this is a case by case basis, and you should not expect to cache every page on your website for obvious reasons, unless there is no dynamic content.

Lastly, you can consider configuring master / slave replication. This will allow you to create n replicas to handle the increasing read workload that you're experiencing. Please make sure you do enough research on how to maintain replication, as it will likely add some overhead to your DB administration tasks

Increasing your database instance size will prove to have diminishing returns. If there is a SQL performance issue I suggest solving this first.