library(foreach)
library(doMC)
registerDoMC(4) #Four-core processor
ti1<-proc.time()
t7<-data.frame(letter=unique(mydf$a),mean=rep(0,26))
t7[,2] <- foreach(i=t7$letter, .combine='c') %dopar% {
 mean(mydf[mydf$a==i,2])
}
eltime<-proc.time()-ti1
eltime