To reproduce:
1. Set up unity-cache-server (if you have already seted up the server go to step 2, if not, set it up first! )
2. On MacOS enter the following command to the terminal: sudo unity-cache-server (this should start the server)
3. Open project attached in the edit
4. Open Edit > Preferences > Cache Server > It should have Cache Server Mode set to Remote and IP address should be 127.0.0.1 (localhost)
5. In build settings change from Standalone to iOS or Android platform or vise versa

Expected result: Platform is switched with Loading Assets are showing
Actual result: Spinning beachball occurs and Unity looks like it froze for about 10 to 40 mins.

Reproduced on:
2018.1.0b5 2018.1.1f1 2018.1.9f1

Not reproduced on ( Unity doesn't look like it froze, but still getting spinning beachball four couple of minutes) :
2017.4.9f1 2018.1.0b4 2018.3.0a9

Crashed on:
2018.1.0a2 - 2018.1.0b1

To setup unity-cache-server:
1. Download and install the latest Node.js from https://nodejs.org/en/download/
2. Download Unity-cache-server from https://github.com/Unity-Technologies/unity-cache-server
3. On MacOS enter the following command in terminal: sudo npm install unity-cache-server -g
this should install the server, make sure you have 6.1.0 version of the server. You can check that by entering this command in terminal: unity-cache-server -V

This time is spent in these lines in AssetImporterUtil.cpp:

for (std::vector<core::string>::iterator s = assetsNeedReimport.begin(); s != assetsNeedReimport.end(); s++)
{
AssetInterface::Get().ImportAtPath(*s);
}

This is the bug fix that may have introduced this particular bug:

changeset: 390482:3bf1c943c950
branch: 2018.1/graphics/bugfix/juhoo
user: Juho Oravainen <juhoo@unity3d.com>
date: Thu Mar 29 14:09:20 2018 +0300
summary: [AssetDB] VerifyAssetsForBuildTarget was checking only assets in Assets folder. Changed this to all assets so packaged stuff get also reimported when needed (case 1019200)

Notes:
2018.2.4f1 it takes a very long time to load the project, but it shows that the project is loading and does not look like it froze.