Do you have a set of sites you frequent on your phone that have ads, but will otherwise function without Javascript? Try using those sites with and without Javascript disabled.
It's a bit depressing how much it drags some pages down.
The loading speed? You can install an ad blocker and enable/disable it. Then force-reload the page. You can also use the developer tools (for example in Chrome) to see the precise timing waterfall.
Could try using a browser add-on like noscript and then only whitelist the website's local scripts, to get an idea of how much faster it loads without the ad network tracking scripts.
It's a bit depressing how much it drags some pages down.