The best way to express a performance improvement is as a ratio of the form $$\frac{T_{old}}{T_{new}}$$Where $T_{old}$ is the time required for the original version and $T_{new}$ is the time required by the modified version.

This goes against the more traditional way of expressing relative changes in terms of percentage. Saying that performance improved by 2.2x is easier to understand than saying it improved by 120%.