Showing the output of onany takes too long for large arrays

[jwahlstrand]

Showing the output of onany takes a ridiculously long time for an Observable wrapping a large array. The example here doesn't need to use onany but is an MWE:

using Observables

o=Observable(rand(3000,3000))

func=onany(o) do val
end

@time display(func)

Output:

1-element Vector{ObserverFunction}:
 ObserverFunction defined at /home/user/.julia/packages/Observables/YdEbO/src/Observables.jl:419 operating on Observable([0.450918 0.330082 … 0.205703 0.00542586; 0.911437 0.605745 … 0.798072 0.804416; … ; 0.859098 0.576516 … 0.951725 0.243679; 0.958703 0.511083 … 0.0163929 0.666014])
 15.139409 seconds (252.63 M allocations: 15.481 GiB, 12.08% gc time, 2.46% compilation time)

This seems to be the cause of JuliaImages/ImageView.jl#292.