Just came to virgin with the oomph bundle, I also have noticed this problem.
I was on giffgaff/O2 with zero delay in receiving messages.
On virgin it is there is definitely a delay after the phone has been inactive on 4g for a while, wifi no issues.
The only difference for me is the sim card, same phone and settings, literally just swapped the sim over so looks like a virgin issue to me.
Live tbb graph