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"500%" is equivalent to "5x"

So "improved by 500%" means "improved by a factor of 5".

In the case of time, improving means reducing.

So time got reduced by a factor of 5.

Which is also wrong. "Improving speed by 100%" means being twice as fast. 100% + 500% = 600%. Better avoid it even if it sounds sooo much more than "double" or "five times".

Improve by itself doesn't imply a direction. It has to be inferred via context. Improving page load time by 500% is perfectly reasonable to say, the reader infers that improve means decrease in this case.

Just as you can improve a golf score, or a marathon pace, or your BMI.

Technically, we can say they increased load speed by 400%. But it would still sound very awkward.

Maybe it takes 1s to load, but now happens 4s before you even think of opening the page?

I guess the velocity measurement "amount of functionality loaded per second" did improve by 500%, a bit contrived though.