It is already in the FAQ (I think @marpet added it), but not yet published:
I’m getting the Error “Cannot construct DataBuffer”. What can I do?
"This error indicates that you don’t have enough memory. Either your system does not have enough memory (RAM) or the configuration for SNAP is not sufficient.
For the SNAP Desktop application, you can increase the amount of memory available to SNAP.
In the ‘etc’ folder of the SNAP installation directory you’ll find a file named snap.conf. Open it in a text editor.
There is the line which starts with ‘default_options=’
In this line you’ll find an option like -J-Xmx5G . Increase the value. You could use something like -J-Xmx13G , if you have enough memory in your computer. By default, it is set to ~75% of the maximum value. This is usually a good choice.
If you experience the error on the command line with gpt or pconvert you need to change different files.
You need to change the corresponding vmoptions files, either gpt.vmoptions or pconvert.vmoptions.
Change the Value after -Xmx in the last line. "
I have just created a new JIRA issue in order to improve the help of the performance parameters in SNAP: https://senbox.atlassian.net/browse/SNAP-1061