Very long clean build (11h) cancelled due to long texture imports

Hey guys.

I have 2 projects building for android, to be run on HTC Focus, with ASTC textures. Clean builds take about 11h then get cancelled, I assume because they are running that long.

Each of the texture reports a warning and has a very long import time, for example:

15130: [Unity] Updating Assets/FRG/Environments/OfficeAndPoliceStation/Props/MaterialsTextures/Airventilation.mat - GUID: 8dc75e3261459024ea747ef45457dc3f...
15131: [Unity] WARNING: RGB Compressed ETC2 sRGB format is not supported, decompressing texture
15132: [Unity] WARNING: RGB Compressed ETC UNorm format is not supported, decompressing texture
15133: [Unity] done. [Time: 667.410149 ms]

Project: https://developer.cloud.unity3d.com/build/orgs/protopixel/projects/fall-protection/

The other android project was also 12h but one time it succeeded, then library got cached and consecutive builds were running fine with 30min build time.
Other project: https://developer.cloud.unity3d.com/build/orgs/protopixel/projects/remind/

Why are texture imports that long? Is this related to ASTC? Haven't tried other formats but also don't want to change format just for cloud build sake. A fresh import on local machine completes in about an hour.

Any help appreciated, thanks.

Edit: it's Unity 2018.3.6f1