Plotting large matrix in R

a = matrix(rnorm(10000^2),ncol=10000)
Ns = c(1000, 5000,10000)
times = data.frame(list(N=Ns))
t1 = sapply(Ns, function(n) {system.time(image(a[1:n,1:n],useRaster=T))['elapsed']})

times$raster <- t1