Search engines (SE) will not index your site if they are forced to crawl large, unoptimized images.
Some backend elements may also be difficult to crawl by search engines. Google, for example, has struggled historically to crawl JavaScript.
Flash and CSS are not always the best resources for mobile devices. This can cause your crawl budget to be strained.
It’s a losing situation in which page speed and crawl budget get sacrificed to accommodate intrusive elements on the pages.
Optimize your webpage speed, particularly over mobile devices, by optimizing on-page resources such as CSS. You can enable caching or compression to make it easier for spiders to crawl your site.