[bronzeage]

The bottleneck of nuclear fusion in all stars is the proton-proton chain reaction, turning two protons into deuterium. Only deuterium can further fuse, and usually deuterium doesn't survive in the sun for long. This reaction is caused by the weak force, so it's extremely slow. This is what's slowing the fusion in all the stars.

"The average proton in the core of the Sun waits 9 billion years before it successfully fuses with another proton. It has not been possible to measure the cross-section of this reaction experimentally because it is so low"

https://en.wikipedia.org/wiki/Proton%E2%80%93proton_chain

In nuclear reactors, we can start from deuterium, making the process much faster and power intensive than the sun.