Eero ad blocking strangling WiFi speeds

To cut a long story short, I am running the modem/router that came from the ISP (Nokia G-2426G-A) in bridge mode and I have a 3x eero pro 6 setup. This seems to deliver reasonable connectivity around the house now. The WAN speed test on the eero app shows 946 down, 204 up.

I've just discovered that turning off Ad blocking in the eero app makes a huge difference. I do speed tests using various apps (fast.com, ookla), but for e.g. using 'speed smart' on iPhone standing next to the gateway eero - with blocking on = 142, 115, 150 with blocking off = 639.69, 716, 693.38 (Mbps down). With blocking off, I've just got 928 down on an Apple TV 4K (wired connection).

This is repeatable and I cycle the network after making the changes.

Any idea what the issue is? I am not currently using any custom site rules on the blocking and there's only one profile which all devices are added to.

I'm paying for eero secure so maybe it's time to ditch that and look at other ad blocking options? 😕