Of course it's faster than developing a browser from scratch, or working with a browser startup on integration etc. but that isn't the question. The question is whether it's even feasible to develop a browser with publicly available information (HTML 5 specs including SVG and MathML, but which one/version? tens of CSS specs and growing as we speak [1], hefty ECMAScript specs, http2+http3/quic, A/V codecs, multiple font formats, etc. etc.). As a glimpse into browser development, yesterday we learned [2] that even a RegExp engine for JavaScript poses a challenge, with specs being "fitted to match reality" eg Chrome behaviour. Honestly, with these results of creating almost a browser monopoly and driving browser development out of existence (Opera, MS), and now even Moz beginning to integrate Chrome code, I don't understand why the clowns at WHATWG and W3C even bother writing specs; they just can't stop it seems. Never has a standardization effort failed so miserably and went so totally out of control, and it'll be a huge problem for generations to come. Talking about almost all human communications (in law, education, politics, medicine, science, culture, personal docs) being held hostage by an ad company; even stone age humans had more liberties in cultural expression.
[1]: https://webplatform.news/issues/2020-06-04
[2]: https://news.ycombinator.com/item?id=23487960
Edit: big congrats to the Flow developers!