[CentOS] CentOS-7.2 USB stick problem

I've installed CentOS-7-x86_64-LiveKDE-1503.iso and CentOS-7-x86_64-LiveKDE-1511.iso on two USB sticks. The first boots up in 90 seconds, but the second takes almost 7 minutes. (I'm talking about the time until my laptop becomes usable.)

After 5 minutes a small window comes up saying it cannot save bookmarks because it does not have permission in the required directory. I assume the delay is a timeout, perhaps due to SELinux?

Once it has started up it seems to run properly.

I wondered if anyone else has encountered this? I'll try installing the minimal version of CentOS-7.2 on the USB stick and see if that is better.