I let SNAP compute the optimal memory usage. I did try setting it myself but it made no difference. FYI, the parameters that were optimised were -Xmx2957m and -Xms2048m. Optimisation of the parameters did not help .
I tried running the speckle filter and also the calibration operation using default settings. Both failed yielding either the DataBuffer or the heap space error which points to a memory issue.
I was trying to process the Sentinel-1 IW data which I downloaded off the ESA hub.