Hi, first of, your FPS are generally low even without the explosions. What hardware are you targeting, mobile or desktop?
With the explosions your FPS go down due to the pixel shader cost of the explosions (so the bigger the screen portion is that the explosion covers, the higher the cost). Since the explosions use translucent materials that means their cost will add up when overlapping. So if you have one explosion, then you pay a certain cost, if you have two explosions covering the same pixel you pay the cost for both, … (if you would have opaque materials then if they would overlap, then you only pay the cost for the material that is on top but nothing for all the materials that are below, with translucent materials you pay the cost for all materials that are overlapping this pixel added together).
Since you need to keep the materials translucent, you have several options that I can currently think of:
(1) reduce the screen portion that is covered by the explosion, so make the explosion smaller
(2) reduce the number of explosions you spawn, so before you spawn a explosion particle system, check beforehand if there was “recently” triggered an explosion in “close” proximity, if so do not spawn an additional one
(3) reduce the complexity of your explosion particle system, so that you get less overlapping translucent meshes/particles