Is there any way to stop this major slowdown from happening?
Not easily. I guess the slowdown is caused partly by the use of
set camera to image and partly by the shader code itself. The first is difficult to deal with without loss of visual quality, and the second is very hard to do anything about. It might be possible to re-write the shader code using some new language features that come with
Dark Shader. For example, the simple blur shader that comes with Dark Shader can be speeded up considerably. It's possible some of EVOLVED's shaders can be speeded up in the same way - but you'd need to buy Dark Shader.
P.S: This is kinda off topic but the OBJECT SCREEN X/Y commands as well as the SET OBJECT TO CAMERA ORIENTATION commands dont seem to work when the bloom shader is on...
Haven't checked this - but I bet the commands DO work. They are probably just not returning the results you want. For example, most of the blur/bloom/HDR shaders prepare one or more initial images of the scene, then process them in the shader(s), then apply them to a screen object (a "screen-aligned quad"). You may find that the commands you mention are giving results related to the screen quad (which is what you actually see) rather than the objects in the scene. May depend where, precisely, in your code you have placed the commands. This is just a guess, not having seen your code or media.