fo <- mpg ~ hp
X <- model.matrix(fo, mtcars)

microbenchmark::microbenchmark(flm=lm <-
                                 lm(fo, mtcars[mtcars$am == 1, ],
                                    weights=rep(1, 13)),
                               flmw=lmw <-
                                 lm.wfit(X[mtcars$am == 1, ], mtcars$mpg[mtcars$am == 1],
                                         w=rep(1, 13)))
# Unit: microseconds
# expr     min       lq     mean   median       uq      max neval cld
#  flm 935.328 951.2735 994.7719 957.2695 1006.128 1333.339   100   b
# flmw  56.895  60.3400  66.2028  64.2940   66.463  186.250   100  a
stopifnot(lm$coefficients == lmw$coefficients)

predict(lm, mtcars)
# [...]
predict(lmw, mtcars)
# Error in UseMethod("predict") :
#   no applicable method for 'predict' applied to an object of class "list"