The other commenter said it well, but just to add an analogy: if you burn a chunk of coal slowly at low temperatures it will last many hours, but if you crush it up a bit and heat it more you can burn it faster. If you blast it at super high temperatures you can get all of your energy in a single big WHOOSH. The total amount of energy released is the same.

The sun burns really really slowly, and only in a relatively small region at the center where it is hottest. The reactors that people are trying to build are trying to burn up the fuel quickly for a number of reasons, one of which is because it’s hard to keep it burning slowly without it sputtering out (to use the same analogy).