Skip Mini Nexus Text
This is only important in a full-game speedrun where you time the run with Real Time Attack (RTA), where every second matters. Therefore we want to cut down every second we can, and in this case we can get free seconds just by a simple concept as skipping having to wait for the text to come up after each Mini Nexus.
When you hit Mini Nexus, meaning you complete the stage, you will have to wait for the text to come up where you then can select to go to the next stage. However it takes a few seconds until the text actually comes up, and we don't want to wait for that. Therefore you simply just, as soon as you hit the Mini Nexus, hold left and spam the "B" button to already select "Got to the next stage" even if the text isn't there yet.
Optimal Time Save
You can only save up to 2.390s when done perfect. This is the optimal time save, only if the you compare two examples where both are done as perfect as possible. If you press "Go to next stage" at first frame it's visible and then compare it to when you use the skip also at the first frame possible, you will get the time save of 2.390s as maximum possible.
Therefore depending how well you mash and time your button press you will get more or less, with a maximum of 2.390s time save per stage.