The latent period never changes no matter the force, why?

Latent period is the period of time between the generation of an action potential in a muscle cell and the start of muscle contraction.
The latent period begins at stimulation and typically lasts about 2 msec. Over this period, the action potential sweeps across the sarcolemma and the sarcoplasmic reticulum releases calcium ions. The muscle fiber does not produce tension during the latent period, because the contraction cycle has yet to begin.

It never changed because increasing the force will not increase the speed of the release of calcium ions.