Final Gather is view dependent and is recalculated for each frame in a sequence. You can store final gather results so that later frames can use the results from a frame rendered earlier to speed up the Final Gather rendering process.
By default all objects participate in photon tracing for global illumination and caustics. Typically, not all are required for you to achieve the look you want. You can specify exactly which objects cast and, or receive photons to reduce the rendering load.