Actually, blur would probably incur a significant slowdown. (I have a background which includes writing image-processing algorithms.)
A simple post-processing effect would be something like color-tint or increasing contrast. Each pixel is sampled once, processed, and output.
A blur filter must examine the surrounding pixels, do an arithmetic mean operation over at least 5 pixels, including a divide operation, to output a pixel value, That’s 5 times the memory accesses (although caching values would reduce that overhead somewhat).
There would definitely be a delay, the only question is whether or not it’s a noticeable delay.