This script can take anywhere from 1.5 to >10 seconds to run on an M1 mac, which is too slow.

It is slow because our GN files spawn python scripts using the exec_script() function, and those python scripts can be slow for various reasons.

The slow scripts can be identified by examining the traces in chrome tracing format that are written under the root build output directory, e.g. out/host_debug_unopt/gn_trace.json.

In many cases, it should be possible to shift these scripts to run as gclient hooks as in flutter/engine#50957 and flutter/buildroot#825.

cc @johnmccutchan