We investigate the coherence of plasma-based soft X-ray laser (XRL) for different conditions that can alter the electron density in the gain region. We first measure the source temporal coherence in amplified spontaneous emission (ASE) mode. We develop a data analysis procedure to extract both its spectral width and pulse duration. These findings are in agreement with the spectral line shape simulations and seeded operation experimental results. Utilizing the deduced spectral width and pulse duration in a one-dimensional Bloch-Maxwell code, we reproduce the experimental temporal coherence properties of the seeded-XRL. Finally, we demonstrate efficient lasing in ASE and seeded mode at an electron density two times higher than the routine conditions. In this regime, using Bloch-Maxwell modeling, we predict the pulse duration of the seeded XRL to be ∼500fs.