When a page is opened in the browser with Crawlera enabled, all the page's resources are routed through Crawlera, i.e. images, stylesheets, scripts, fonts, requests to CDNs, traffic analysers, advertisements, etc. And since Crawlera adds throttling to each request (by design, to perform its main purpose of "crawling politely" and avoiding bans), loading a page in the browser can indeed take a significant amount of time.
To speed up the loading time (while also cutting on the plan usage quota), it's recommended to exclude the aforementioned static assets from being processed by Crawlera. For instance, in crawlera-headless-proxy tool, AdBlock and Direct Access features can be enabled to filter out the unessential resources.