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.

Opera actually has this ability built-in. It lets you A/B test websites (with/without ads) by going to the URL "speedtest".

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.

You'll also need to average out the speed of your website accessed from different geographical locations.

You can try Pi-Hole which stops the ad and tracking DNS requests from getting through.