Here is a basic format to follow:
Make sure all images are the lowest resolution before they become ugly. Also there is a new web image format that came out and I need to look that up.
Make sure your html markup matches the latest SEO friendly formating. H1, H2 tags have proper content. Think about how google shows the intro to your link.
Make sure the html code is formatted correctly. Like having alt tags for images and all tags are being closed.
Make sure your javascript files are limited. Bigger files are better than 100s of smaller ones.
Make sure your site isn't calling tons of 3rd party services that could be slow and make the page load slower because it is waiting waitng for something like autotrader to fill in that information.
Use old school html pages as much as possible. This means you really need to limit what is coming from a database for your CMS base pages.
The tricky part comes in for loading your vehicle listing pages. Many platforms and "ecommerce" shops use the database to build these pages. Depending on the programming language this can be fast but many developers will use an automatic transmission instead of a manual one. The automatic is obviously tuned to a certain degree but can't handle power shifting.
Many software companies will build an LS engine at first but then throw on Amazon parts as the years progress and barely will change the oil. This is called tech debt and companies don't want to pay for a rebuild. They want developers to make it work and depending on the level of the mechanic, you get what you pay for.
If you can give me an idea what the engine is like, I can give you some better feedback. I wish it was like looking at 2014 Ford Mustang where you can guess to what the power plant is. But many times it is not that easy. It also gets worse if you had a mechanic build you that engine without using a brand engine block.
Did you buy one of those built sites from a "autodealer" vendors? IF so, you might not have much ability to fix it and the company will likely not want to take on tech debt for a small company.
