[jitter] Implementing a 1D difference equation in Jitter

Is there a simple way to implement difference equations with feedback in Jitter such as:

y[n] = (1/2)*(y[n-1]+x[n])

where y[n] is the output, and x[n] is the input. 

I've been able to implement such using jit.iter and a math patch, but I can't find a way to put the data back into a jitter matrix without a large CPU hit. Right now I'm using setcell commands to fill a matrix and it takes 10x more CPU than the filter itself!

Here's my processing chain broken down:

input->jit.iter->difference equation patch->jit.matrix setcell commands->jit.graph

What can I do to get rid of the CPU hit from all the setcell commands?