I have done a serious research on this topic and i found that all device has an opening lag and it is independent of your camera applications i.e, it is hardware dependent. It(the lag) will be keep varying depending on the memory and processor state. Some smartphones are successful in covering it with their high end hardware resources.
Consider a scenario : if we have a bundle of apps opened in memory and you opened camera with any of your camera application then you feel lagging more than before. Now you can understand easily that this lagging time is a varying factor.
i have even created an application for this called "camera opening lag checker" it calculates your camera lag time and allows you to compare it with world's top devices real time.
Meizu PRO 5's camera opened in .17ms where Samsung s6,s7 are also performing great with .25ms, .26 respectively