While this paper is quite easy and occasionally funny to read, being a total math idiot, I need somebody to help me out how to read the benchmarks at the end. T

While this paper is quite easy and occasionally funny to read, being a total math idiot, I need somebody to help me out how to read the benchmarks at the end. The author claims that on average converting stack-based bytecode to register-based bytecode results in an average of 25% performance improvement, I have trouble finding where that number comes from. The charts are said to be a comparison of optimized code relative to unoptimized code, my question is, how come the unit on the y-axis is a percentage, and the unoptimized code isn't used as a base-line all labeled as 100% or given an absolute average time-took? The tiny gaps between unoptimized and optimized code are confusing me.