N<-10000
n<-20
dd<-data.frame(x=runif(N))
dd<-transform(dd, y= 4-2*x + rnorm(N))

fits<-t(sapply(by(dd, rep(1:(N/n), each=n), function(x) lm(y~x, x)), coef))
head(fits)
#   (Intercept)          x
# 1    4.025626 -2.3476841
# 2    4.684731 -3.0566627
# 3    4.011690 -1.8731735
# 4    3.788382 -1.9182377
# 5    3.461123 -1.0965173
# 6    3.671282 -0.9247785