Speeding up band math in R 'raster' package

I have some GeoTiff rasters with over 100 bands in total, but they are not huge: resolution is about 1200x600 and total size is less than 7 mb. I'm trying to do a simple NDVI calculation:

raster <- stack('path_to_raster_1', 'path_to_raster_2')
red <- 60 # red band number
NIR <- 75 # NIR band number
f_NDVI <- function(x){
  (x[NIR]-x[red])/(x[NIR]+x[red])

}
NDVI <- calc(raster, f_NDVI)

The calculation runs extremely slow and takes about 10 minuets.

Is there a way to optimise it and why is the calculation so slow in my case?

Answer

I expect it will be much quicker this way (using a vectorized approach in the NDVI function)

f_NDVI <- function(x) {
  (x[,1]-x[,2])/(x[,1]+x[,2])
}


# subset the two layers you need
s <- raster[[c(60, 75)]]

NDVI <- calc(s, f_NDVI)