Hi All,

I'd like to know why is plotRGB() so fast compared to using plot() ?

```
library("raster")
library("microbenchmark")

x = raster(matrix(runif(300**2), 300, 300))
x = stack(x, x, x)
bench = microbenchmark(
  plot(x),
  plot(subset(x, 1)),
  plotRGB(x, scale = 1))

bench
```

I get medians around:
plot(x): 1480
plot(subset(x, 1)): 501
plotRGB(x, scale = 1): 212

plotRGB is more than 6 times faster and it improves with raster size in my
experience.


Etienne

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