In the past there was a discussion about ethernet speed between internal network and when using Firewall and different subnets. I can confirm there is no speed downgrade when using multiple VLANs.
I am using tagged VLANs that are ingressing as tagged to the omnia from currently 3 managed network switches.
I am using really simple devices. Omnia as client of iperf3 and Raspberry PI 4 4GB with hardwario hub slightly customized raspbian.
You can even see strangely that intervlan is faster.
Therefore I think that those that say that Omnia is slow device are either having some broken device or have different problem or are lying. I have original IG Omnia. Newer one I’d say are 1% better as I dont remember due to what but newer hardware revisions are slighly better. So if my older omnia is good, then newer omnia will be only better. I dont use WIFI on omnia at all, maybe that could make the device slower, but I doubt about it. I have around 30 devices on the network but running only normal amount of traffic to the WAN and probably way more than normal traffic internally due to backups and NAS