Question about mekanism's speed upgrades

Speed upgrades makes the machine consume more energy to finish a task, or they just speed up the consumption?

I tried to search about it, but i still dont understand.

if it increases the base energy consuption, then 1000 KJ would not produce as much results, and would be necessary for me to avoid using them if i want 100% efficiency.

If it just speeds up the base energy consuption, then its actually more efficient to use speed upgrades. Same eficiency, lesser time.