Opencart speed Version 2.3.0.2 with many of categories/products

We have recently added a lot of products and categories to our opencart 2.3.0.2 store and have found that the website has slowed down substantially. In particular, the page load speed of products and categories as well as the search speed has become very slow.

Our store has 656 categories and 1616 products at the moment.

I am looking for suggestions on how to improve the page load and search speed. We are on a very good VPS server, so it is not the server that is the issues. I am looking for tweaks to the code or configuration that would help.

Thanks for your help!

This question was asked gazillion number of times. You need to add indexes to your database. Use search to find a solution.

... and cache your category menus

Well, at first, you'll have to show us some Results, at best, by testing a
20-30 Product Category Page, in Mobile and Desktop View, by use of
http://gtmetrix.com
to find out, in detail, what needs to be done, to make sure, to release
all brakes, resulting out of a so-called Default Installation, before talking
about additional steps. Everything else is plain Theory
Ernie