I really dislike the term "up to N%" - this reminds me of sleazy commercials reporting "up to 80% discount". 0.2% is also "up to 100%". Can't more accurate figures be reported?
Or wouldn't you say that a car that goes 100 mph is 100% faster than a car that goes 50 mph?
It's all in how it's worded. Instead of being worded as "100% improvement", it could have been worded "50% decrease in how long it takes to complete the operation". One of these rolls off the tongue nicer, but both are valid and mean the same thing.
In your example, both the sales prices are worded as a negative value. "50% off" could be worded as a positive value as "100% more". In this case, LibreOffice has 100% more than the original amount of speed. Twice as fast.
I rather use average improvement rather than best case improvement - that way users don't feel duped.