dat <- data.frame(
  a = runif(1000),
  b = runif(1000),
  c = runif(1000),
  group = factor(rep(c(1, 2), 500))
)

library(dplyr)
dat %>% group_by(group) %>% summarise_all(mean)
#microbenchmark = 7.1 milliseconds

library(doBy)
summaryBy(. ~ group, dat, FUN = mean)
#microbenchmark = 4.6 milliseconds

library(data.table)
setDT(dat)[, lapply(.SD, mean), by = 'group']
#microbenchmark = 1.8 milliseconds