There is an user's guide for talking about how to build the Android BSP.
https://www.nxp.com/docs/en/user-guide/ANDROID_USERS_GUIDE.pdf
Basically, please read the Chapter 3.1 in the usr's guide.
the i.MX Android release source code consists of three parts:
- NXP i.MX public source code, which is maintained in the CodeAurora Forum repository.
- AOSP Android public source code, which is maintained in android.googlesource.com.
- NXP i.MX Android proprietary source code package, which is maintained in http://www.nxp.com.
You may check which part of the source code is downloading slow. You may use 'proxy' server setting on your host for speed up the download.