In nearly every subfield of computing, along nearly every metric, most people would kill for a 20% improvement.
A processor 20% faster would dominate the market for years. A new compression algorithm that was 20% smaller would be either copied or used by every archiving system. And yes, a website reduced by 20% is significant.
This kind of arithmetic is always very unclear and causes all kinds of confusion / miscommunication. This is why I always prefer to be explicit about the absolute numbers (maybe in addition to the relative percentages): "it used 100 mb and now it uses 25 mb (which is a 300% reduction when looking at the final result / which is a 75% reduction based on the initial result)".
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