When a page is opened in the browser with Zyte Smart Proxy Manager(formerly Crawlera) enabled, all the page's resources are routed through Smart Proxy Manager, i.e. images, stylesheets, scripts, fonts, requests to CDNs, traffic analysers, advertisements, etc. And since Smart Proxy Manager 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 Smart Proxy Manager. For instance, in crawlera-headless-proxy tool, AdBlock and Direct Access features can be enabled to filter out the unessential resources.