Optimization is the process of ensuring that only the scripts/styles are loaded that are absolutely necessary to fully and correctly display a page of the website in order to not waste any resources. The result is usually a faster loading website.
I believe that Optimization and Localization are much more intertwined than most people might think.
As an example we take a bilingual website in English and Japanese.
For the English font a random Google font is being loaded. Japanese looks best with its own font and there are many font sets available for it. Easiest would be to simply load the Japanese font if a visitor is viewing the site in Japanese, but what about the random Google font? It is not necessary to load that too, is it?
With Optimization done properly, the Latin based (Google) font is loaded exclusively for English and the Japanese font exclusively when the site language is switched to Japanese. That way we can ensure to keep the loading of resources limited to the very minimum.
There is no official abbreviation for optimization, but since there are i18n and l10n, for fun we can say: o10n.
Similar as with the Localization services, the best way to start Optimization is to have me do an Assessment of the site.
And although a lot can be discovered from the frontend, that would not paint the whole picture. Therefore I also ask backend access.
During the Assessment I check for potential plugin conflicts and I also have a look at the theme code.
The reason for the Assessment is that no website is the same. There are thousands of themes on the market and even more plugins. Many of them come at a premium, so it is impossible to see any of the code on beforehand.
And on top of all of that, there is hosting which also can be an influential factor.
The combinations are endless, so it basically is impossible for me to give a reliable answer to questions without being able to have a thorough look at the backend of the site.
The advantage of the Assessment is that I always manage to discover many of the (potential) problems in a relatively short time.
After the Assessment I write a full report with recommendations and I include an estimate for the next steps to take to solve the issues.
The report itself is a valuable document that can be used as a guide for future work too.