With a little extra effort, you can write the summing loop such that only a certain number of the values are added per scan, for instance, add 10 number this scan, 10 more the next until you are done.
You would generally tic-toc back and forth between two total sum registers. One would hold the previously finished sum and the other would be the sum you are currently working on.
Of course, this means that the active sum would only update every 10 scans or so... but each of those scans would be faster.
It all depends on what you require.