Masking Performance

We are using the Combiner.ApplyMaskTransform() but are finding it to be rather slow - even for reasonably small images, the masking is taking around 1 second to perform. This is a problem when what we need to do is carry this out over 4 images and then render them together - the time adds up and before long, the full image generation takes 6-7 seconds. Of course, as the images go a bit larger, this gets worse still. Is there anything we can do or should be aware of that will increase or decrease the performance of this method? At the moment, for instance, the amount of transparent (black) mask is quite high (around 90% of the image). Alterntively, is there a faster, more performant method for masking anywhere in Graphics Mill?

Many Thanks in advance!!