Does it change meaningfully under load? As long as the API p50 / p95 latencies are what you expect, the HTML generation / rendering is rarely a problem.
Most of the raw HTML generation, rendering is done by frameworks and browsers respectively and they're pretty standard / efficient pieces of software.
I guess we all come at this with our own assumptions. When I think about testing, I'm also thinking about the base experience, not just adding resources and adding load.