Computationally Inexpensive Imputation Techniques in R
Take a look at the h20 package https://cran.r-project.org/web/packages/h2o/h2o.pdf. 

Everything is designed with parallelization in mind. I've had great success with many of their implementations, in R and Scala. 

If you have to do it in R and are going for pure speed I doubt you'll find something faster.