Not trying to defend Chrome here as I dislike their other behaviours, but just from what’s presented in the video, an alternative explanation would be caching. That is, when the reloading is triggered by the switch of user-agent, the cache is reused and thus a shorter load time.
To exclude this effect, the user needs to either
Spoof the user-agent and at the same time clear cache (you can disable cache when reloading through the developer’s tool), or
Clear cache, spoof the user-agent to Chrome. Load page, disable the spoofing, reload.
Not trying to defend Chrome here as I dislike their other behaviours, but just from what’s presented in the video, an alternative explanation would be caching. That is, when the reloading is triggered by the switch of user-agent, the cache is reused and thus a shorter load time.
To exclude this effect, the user needs to either