[CentOS] DELL U2412M monitor slow boot

Using large Dell U2412M monitor here, with CentOS7. It is really slow
to boot up, each time it would hang after the following messages for a
very long time, with no logs, then it would continue to boot up fine:

[  OK  ] Reached target Initrd Default Target.

Welcome to CentOS Linux 7 (Core)!

This does not occur for other Dell monitor like DELL P190S.

Any ideas?