library(raster)

## Loading required package: sp

library(parallel)

setwd('~/Downloads/03')
cores <- 3

pliki <- dir(pattern = "tif")[1:100]

indeksy <- splitIndices(nx = length(pliki), ncl = cores)


policz_sume <- function(x){
  sum(stack(pliki[x]))
}

# system.time(mclapply(indeksy, FUN = policz_sume, mc.cores = cores ))
# system.time(sum(stack(pliki)))

r <- mclapply(indeksy, FUN = policz_sume, mc.cores = cores )
r <- Reduce('+',r)
plot(r)