Unlike linear regression with normally distributed residuals, it is not possible to find a closed-form expression for the coefficient values that maximizes the likelihood function, so an iterative process must be used instead.
Wikipedia
You can find closed-form expressions for SOME likelihood functions. This is the case for logistic regression.
Anyway, the takeaway that I think you were downvoted for omitting is that if you can bypass the serialization, startup, and shutdown steps associated with the traditional Hadoop iterative process you'll get a huge speedup.
