Why is the rate of primary succession slow as compared to secondary succession?

Secondary succession usually occurs faster than primary succession because the substrate is already present. Compared to the image below, secondary succession does not need to have pioneer species colonize the land nor does decomposition need to occur to create a layer of topsoil.

Primary succession occurs on land that lacks soil. When pioneer species colonize such land, they break the rocks, and the process of soil formation begins. Soil formation is an arduous process and takes many years. On the other hand, secondary succession occurs on land where soil is already present. Moreover, roots and seeds of few pants are also present in the soil. This helps in reestablishment of plants and other species. Therefore, the rate of primary succession is slow as compared to secondary succession.