extrema(skipmissing(cube)) much slower than extrema(skipmissing(readcubedata(cube)))

[felixcremer]

When I am trying to get the range of the non-missing values it is much faster to read the whole dataset into memory. For only extrema without skipmissing it doesn't make a difference. These timings are on a 15000x15000 array that is on my local computer.

julia> @time extrema(skipmissing(readcubedata(clocal76)))
  0.878060 seconds (148 allocations: 891.717 MiB, 3.05% gc time)
(0x00, 0xfe)

julia> @time extrema(skipmissing(clocal76))
 56.791334 seconds (900.00 M allocations: 72.180 GiB, 18.44% gc time)
(0x00, 0xfe)

julia> @time extrema(clocal76)
  0.827030 seconds (98 allocations: 891.714 MiB, 1.68% gc time)
(missing, missing)

julia> @time extrema(readcubedata(clocal76))
  0.842818 seconds (141 allocations: 891.717 MiB, 5.14% gc time)
(missing, missing)

extrema is only the motivating example, but with a simple collect(skipmissing(cube)) I get very similar timings. I suspect that skipmissing somehow indexes into every single index.