Low Cell Density, and Slow Growth and Cell Proliferation Rate

Why when cells are seeded at low density, could there be a retardation in cell growth? I've seeded human fibroblasts at low cell density and they grew very slowly. What is the reason behind it? No cell signaling?

How can calculate cell proliferation rate? If I use DAPI staining, then I would have to kill the cells, is there any other way?