Android Question Class or Activity, what is better?

Does each activity handle a different screen? If yes then two activities sounds like a good solution.

Yes, the first activity generates the screenshot and the second activity analyses this screenshot and returns data to the first activity which then updates its screen. The second activity is only normally needed once.

I just wondered if there would be any benefit (speed or comfort) from using a class instead of the second activity,as it it really does is maths.

A footnote. When the app is working correctly, the second activity will not even show a screen, just a progress dialog will be shown from activity 1. At present I am showing the calculations on screen for debugging.

Then maybe a service could be good to do that calculations.
From the service you can raise subs in your main which then updates the ui (progress)