> but importing two packages such as DifferentialEquations and Plots, on my very modern machine takes ~30seconds (this is after they've been "precompiled").

FWIW, it is quite significantly improved in the upcoming 1.6 release.

1.5:

    julia> @time using Plots
      8.859293 seconds (15.99 M allocations: 913.783 MiB, 3.86% gc time)
    julia> @time using DifferentialEquations
     25.394033 seconds (58.78 M allocations: 3.222 GiB, 3.96% gc time)

latest master:

    julia> @time using Plots
      3.957724 seconds (7.67 M allocations: 537.424 MiB, 5.50% gc time)
    julia> @time using DifferentialEquations
      9.708388 seconds (23.08 M allocations: 1.537 GiB, 6.20% gc time)