https://unity.slack.com/archives/C06TQE31T/p1512489721000065
https://unity.slack.com/archives/C06TQE31T/p1512489857000593

Basically, the problem is, repacking will happen for each asset bundles. If user has 3 asset bundle, we will pack once for each building of asset bundle. But packing only happen for first bundle, the rest will skip because we packed and cached.

What the user observe is progress bar of packing (even skipping, the "comparing hash..." will show like in Mac while entering play mode) and message like

Packing sprites:
[ 77 MB ]- Skipping packing - Sprite Packer (Legacy) is off.
[ 77 MB ]Sprite Atlas Operation : " ## Packed All Sprite Atlases ## " took 0.035174 sec (current mem: 77 MB)

in the editor log. What I am going to fix is just get rid of the progress bar and logs print out.