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.
Generally 100% is used in terms of an increase, where 50% would be used in terms of the same decrease. Since it's twice as fast, it's 100% faster. Which also means it takes 50% less time to do the same task. If it had gotten slower, it would be 50% slower, which means it takes 100% more time to do the same task.
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.
