Website speed and speed Index improvement

Hello there,
One of my clients Free Ads website designed in laravel. We want to optimize its speed. We were testing site on insights and getting Speed Index 14.7 s. Every time i reduce css or js website malfunctioning . Can anyone tell us any tool or service that can use for reduce unused css and js.
thank you in advance

Why do you think the problem is with the css and js?

For instance, the site has a large number of embedded SVG files. These all have to be parsed before the website can appear - what happens if all of
these are delay-loaded from external files?

Many thanks,I see , is there any best practice to use svg ? or is it better to use images instead of svg?