Other than Google Streaming / Stadia and some AMP sites I have never seen a Google site / app load in less than 2 seconds or so (which is kinda slow, all things considered). So I'm a little confused about this latency sensitive environment statement.
Sure, what you say is correct in that using agent strings can provide an optimized bundle to the browser the faster, but isn't there far better optimizations that Google could make first? I've seen and worked on very fast, sub second web apps before so this argument feels more speculative than any good reasoning.
Besides, you shouldn't be gating _so hard_ with agent strings anyway. The non-Chromium version of Edge is probably fine on YouTube as well. You likely want to gate away the obvious offenders, assume the best with the newer browsers but allow a fallback option that detects a few things on load so you can adjust and set cookies for the next time.