Which processing point to do speckle filtering?

dear all
i was wondering that, usually at which point is the best to use the speckle filtering?
especially if you are trying to perform terrain flattening. before? after? or in the very end after terrain correction?

please give me some ideas
or guide me to any references you might seem that it is related

thank you so much

Strictly, speckle filtering should be done before terrain correction (including flattening) as the underlying statistical assumption of most speckle filtering routines relate to the intensity data in SAR geometry.

thank you !
ill process with my data as so then