R help - Jan 2014 - Why is kmeans incredibly slow on this Ubuntu machine?

X is a 5000 x 5 matrix (numerical variables)
k = 6

My desktop machine is a Xeon-processor Dell Precision T3500.

[Ubuntu 12.04.4 LTS (GNU/Linux 3.2.0-58-generic x86_64) with 24 GB RAM]

[R version 3.0.2 (2013-09-25) -- "Frisbee Sailing"
Copyright (C) 2013 The R Foundation for Statistical Computing
Platform: x86_64-pc-linux-gnu (64-bit)]

 > system.time(X.km <- kmeans(X, centers=k, nstart=25))
    user  system elapsed
  49.763  52.347 103.426

Compared to a Win64 laptop with a similar R version, and the same data:
 > system.time(X.km <- kmeans(X, centers=k, nstart=25))

    user  system elapsed
    0.36    0.00    0.37

Almost instantaneous.


Is there something obvious I'm missing?
Ben