First. Martini uses reflection, it's IMPOSSIBLE to make it as fast as Gin without removing all the reflection. Obviously it would break all the API, it would not be Martini anymore.
Martini is not slow because a bug, it's slow by design.
Second, Gin uses the fastest http router available, HttpRouter. I strongly believe that people should use HttpRouter, it will work perfect for you unless you need regex to validate the URL. The problem is that HttpRouter is not strongly featured, it lacks things like groups, middlwares, error management, control flow, rendering...
One requirement for my startup was high performance, HttpRouter was the best choice, we added a very lightweight system on top of it, so developers are happier.
The final results, from 20x to 40x times the performance.
As I said, if you need performance and productivity Gin is probably a good way to go :) I hope it was useful.