Create NDSI GeoTIFF from Sentinel 3 with python problems between *.dim file and *.SEN3 file

Hi everybody!
From couple of weeks I tried to find a solution to create Snow Extent maps. Right now I am in the point where i must choose a solution and to test that solution on some 15-20 S3 scenes. One solution is with NDSI that I created with python (here is the script). (7.3 KB)
For testing initially, I decided to go with a *.dim file created in SNAP and everything is working smooth, the procesing is around 3-4 minutes what is excellent.

My problem now is when I load the *.SEN3 file (it’s opens the xml schema I think, maybe my explantion here is a bit clumsy), subset it to the right dimensions just ones of that *.dim file (I added a subset operator with the geoRegion approximate of the dim extent) and run the whole process, processing is simply non efficient when it writes the GeoTIFF.(writes 5-6 rows of the NDSI in about 3 hours)

Anyone knows what is the problem that it’s causing this??