KMeans doesn't work in station laptop RAM 64 G CPU E5-2620v3@2.40GHz?

Hi I want to classify my data into two class (1 or 2) I choose the ratio of training 70/30 and I have 7148 images ( 3501 samples for 1st class & 3647 samples for 2nd class) ... I extracted the local feature for all images and I encode it by Bag of Visual Word (BOW) to represent the global features... So to extract the visual word we used KMeans and the input of KMeans is an array 1208012*256 and K = 5000, but the execution of KMeans is very long most than 12 hours and the laptop blocked without any result, by the way, I use Matlab2017b on a laptop has 64 G of RAM and CPU E5-2620v3@2.40GHz...I'm confused what's wrong because when I try it with K = 1000, it's work without any problem ..please help me