I don't understand that a product oriented for software engineer use sentence like "Significantly improved startup time" instead of real numbers. What does that mean "significantly"? 10%...50%?
like lots of software, a lot of that can depend on plugins, themes, how many buffers are getting re-opened from the previous editor state, etc.... I imagine it makes it hard to produce meaningful numbers outside of extremely controlled test cases, and might be misleading to some people.
That would probably be a good middle ground. You will still get people saying "what the hell, mine take 3 seconds to start up with 10000 plugins and 20 1MB files open, but you say 350ms?" But, can't please everyone.
I think values like that are more often used by advertisers, instead of software engineers honestly. It's incredibly tricky to measure the increase of start-up time as a percentage since it matters so much on hardware, what files you have open, if it's cached, etc.
Using a single percentage would probably be more misleading than just saying "significantly improved".