First run of st_apply() is several times slower than second

[kadyb]

Have you observed that the first run of st_apply() is several times slower (3-4) than the second and subsequent runs? Generally in R, the first run of functions is usually slower, but the differences are a few percent, but in the case of st_apply() it is very noticeable.

library("stars")

n = 5000
bbox = st_bbox(c(xmin = 0, xmax = 1, ymin = 0, ymax = 1))
r1 = st_as_stars(bbox, nx = n, ny = n, values = rnorm(n^2))
r2 = st_as_stars(bbox, nx = n, ny = n, values = rnorm(n^2))
rr = merge(c(r1, r2))

f = function(x, y) {(x - y) / (x + y)}

system.time(st_apply(rr, 1:2, f))
#> 4.20, 1.26, 1.38

## for comparison
system.time(f(r1, r2))
#> 0.17, 0.17, 0.17