Sandbox slowness on OSX

### Description of the problem / feature request:

building has been extremely slow with the default darwin-sandbox

### Bugs: what's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

A mini repro could be found in https://github.com/alexeagle/rules_sass_repro
The repro contains 40 empty sass files
Running the sass compiler on them should be fast

bazel build :all takes ~60s on my mac

bazel build --strategy=SassCompiler=local :all takes ~4s

### What operating system are you running Bazel on?

Mac OS 10.14.4

### What's the output of `bazel info release`?

release 0.25.0

### Have you found anything relevant by searching the web?

I found these issues: https://github.com/bazelbuild/bazel/issues/902 and https://github.com/bazelbuild/bazel/issues/1836 but they all seem obsolete.

### JSON profile

According to https://docs.bazel.build/versions/master/skylark/performance.html#json-profile, I grabbed profiles for different strategies:

[profiles.zip](https://github.com/bazelbuild/bazel/files/3140874/profiles.zip)