A: We just ran some tests and wanted to share the results.

We tested 2 sites (both already optimized for pagespeed with the host Nestify.io).

Site #1 is a brand new Wordpress site, a few plugins, no other scripts on the site.

Site #2 is an established 6-year old WordPress site with other scripts in the header and a variety of plugins installed.

Main findings: if you have other scripts on the site, it likely will not take extra time as our script loads asynchronously. If you run an extremely lean site, it may add about ~0.4s to the TTI metric as shown below. It will probably be less if your site takes longer than 0.2s to load as the script will load asynchronously.

SITE #1

Before (average of 3 tests):
DOM Content Loaded: 0.17s
Time-to-interactive: 0.18s

After (average of 3 tests):
DOM Content Loaded: 0.16s
Time-to-interactive: 0.58s

SITE #2

Before (average of 3 tests):
DOM Content Loaded: 0.5s
Time-to-interactive: 1.86s

After (average of 3 tests):
DOM Content Loaded: 0.45s
Time-to-interactive: 1.39s

OUR TAKEAWAY:
This wasn't the most perfect scientific test, but hopefully shines some light on page speed. I think most established sites will be relatively unaffected as they'll have other scripts in the header they need to use as well and generally more established sites take a bit longer to load anyways. That said, we are always looking to improve our loading speed on our side so it will likely only get better from here.

We have quite a few things we know we can optimize, and will be publishing some optimizations in the next week (although this optimizes our database side, so I'm not sure yet if it will improve page speed for users). We are always keeping an eye on page speed though.