Dithering is MUCH less expensive than translucency. Translucency also gets increasingly more and more expensive with each layer of it you are looking through, 3 layers of lit translucency can be extremely expensive depending on how large it is on screen.
In regards to the noise, there’s not really a good solution for this as it’s just how dithering works. By setting randomness to zero you can make the dithering a fixed pattern but you won’t be able to fully eliminate it. Disabling TemporalAA can also eliminate any smearing you may have.
You could try using full translucency with the solution MostHostLA posted above, but I’ve not tried it so I can’t comment on how involved it is.