Unsupervised Classification delay even form subset images

I’m trying to classify (unsupervised classification EM algorithm) a subset of a SETNINEL-2 image using 4 bands (B2,B3,B4,B8 -10 meters ), in 6 thematic classes. Image size about 100 MB. It lasted about 15 minutes!
The processor of my PC is intel-i7 with 32 GB ram and SSD hard disk.
Is it normal ? Have I done something wrong ?
Ioannis Faraslis