Perhaps use chrome's lighthouse thing to measure time-to-interactive too?

[fitzgen]

Motivation

If we can easily measure time-to-interactive / page load -- which is the real thing we care about when we measure code size -- then we should do it.

Proposed Solution

Use the chrome lighthouse tool with https://developers.google.com/web/tools/lighthouse/audits/time-to-interactive

Alternatives

If we can do this through webdriver easily, we should prefer that because it isn't chrome-only.