I've always had a problem with this too. If a product is 80% off, you pay 20% of the original price; 100% off and it's free. If a product is 80% faster, it runs in 20% of the original time. If it's 100% faster, it runs in zero time.
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.