The bug

My game takes over 30 seconds to reach the loading screen, and I just recently figured out the cause: packs in the development_behavior_packs and development_resource_packs folders. The more files you have in the pack, the slower launching the game becomes.
It doesn't even matter if they are legal pack files or not, so it's not parsing or validation that's slow. In fact it doesn't even matter if the folder is a valid pack – removing the manifest file doesn't prevent the stall.
Note that the normal behavior_packs and resource_packs folders don't appear to have this issue, however using them is not an option for me because closing and reopening Minecraft entirely in order to reload changes is a non-starter.

How to reproduce

1. Download the attached behavior pack. It has 66,000 extremely small files in it
2. Unzip it in the development_behavior_packs folder
3. Launch the game

Expected behavior
It launches in under 2 seconds with a modern PC

Actual behavior
You're staring at a gray screen for multiple minutes