So, pipelining mode is only usable if you are using the SSH connection type (-c ssh), so I'd first check to make sure you are running on a system where that is the default -- i.e. not Enterprise Linux (RHEL, CentOS, etc).
You've mentioned you are using RHEL, so this has to be explicitly requested, which is not the default -- and due to lack of ControlPersist on this platform (which is why -c ssh is not the default there but is everywhere it's available) it may not be as performant.
So, by default, your EL system is still using paramiko, which I hope explains things.
The other case is if your playbook uses "su" (not sudo), in which case pipelining is explicitly disabled due to technical limitations.
(I'd recommend maybe considering running from a Fedora -- or even Ubuntu -- host if you want it full-on optimized)