With the L8X PLCs there isn’t a system overhead time slice anymore as the comms are now broken off into a separate CPU on that PLC itself. Before the comms had to share resources on the same CPU as the processing of the PLC but with the L8s that isn’t the case.
Are you using AOIs or UDT? If AOIs then you will probably want to start down the path of breaking the data that you want in the HMI into UDTs. See this long discussion: https://forum.inductiveautomation.com/t/device-load-factor