quadratic latency for pasting big expression into REPL

[adienes]

str = "f(x, y, z) = 1"
for i in 1:2000
    str *= " + $(rand()) * $(rand(['x', 'y', 'z']))^$(rand())"
end
str |> clipboard

then pasting takes several seconds; this appears to scale quadratically with the length of the expression. reproduces on old versions too so this is not a recent regression

[oscardssmith]

This likely needs a profile to figure out where the time is being spent.