With sprite objects that are not doing anything, Construct 2 is approximately as fast as a native engine. So you are either hitting the limit of your hardware, or these sprites are actually doing something (perhaps collision testing?).
Are all those objects on-screen? If so, that's a lot, but a desktop computer should be able to handle it no problem. If they're mostly offscreen it's definitely not rendering that's the problem, and again suggests it's the logic. If you share your .capx, I could take a look and comment further.
Applying effects to lots of individual instances is pretty inefficient. It's way, way faster to apply a layer effect, and put all the objects on that layer, if it's possible to do it that way. For example if you can put all your shadows on a single layer with a blur effect, that should run pretty well.