Translating Proc Mixed into R

Dear All,
I am having trouble in translating the following SAS statement for Proc Mixed into R.

proc mixed data = b;
class period;
model dh = period/s;
repeated period / subject = patient type = unstructured r rcorr;

Here is some additional information from SAS output that might help.
Data Set WORK.GH1
Dependent Variable DH
Covariance Structure Unstructured
Subject Effect PATIENT
Estimation Method REML
Residual Variance Method None
Fixed Effects SE Method Model-Based
Degrees of Freedom Method Between-Within

I guess my first question is, which R package should I use? I have tried lme4, but it is frustratingly slow, even for a small dataset (18 obs). The second questions is translating the SAS code so that I can replicate the output.

Any help will be much appreciated!
Thank you,