@larkim how much to expect depends on a couple of things.

You said you get full speed connected to thye main, correct?

If you are connected to a node just do the math, the M4 is can sync at up to 867mbps.

So if you are close (and often even if your a smallish distabce away) your device may sync 867mbps so lets assume that.

Then, when you communicate to the internet you go via the node meaning the packets are reveived and retransmitted by the node which means whatever the throughput is it will be halved.

Next you need to account for the wireless protocol overhead which often means around a 30% cost.

expected throughput = (867 * 0.7) /2 = approx. 300 mbps.

So your getting a bit lower than you should be getting, might besyncing at a lower bit rate, you should be able to check on a PC.

Also this assumes the device you are using can do Wireless AC, you should check, again should be possible on a PC

  @raven-au  Much appreciated, thanks for that info.

Yes, I'm getting full 300Mbps when wired up, but the most I've seen so far (testing wirelessly on my Pixel 7a right next to the main Deco) was 185Mbps.  So I've got some tweaking to do maybe?

Wonder if it is congestion (will run the optimiser) or whether the phone has selected 5Ghz or 2.4Ghz bands?  Or maybe there was other network traffic at the time.  I will also try via a laptop as I'm conscious a phone may not want / need to get full speed anyway.

I presume the other factor is whether the satellite M4 units are using bandwidth too?  

I've got mine stacked linearly (i.e. Main feeds unit 1, unit 1 feeds unit 2) because of the geography of my house.  I am on the lookout to get a fourth unit which may help with that as I assume (I may be wrong) that optimal is more of a star-configuration, rather than a daisy-chain of devices.  Not that that is impacting the current discussion, as I'm only relying here on the main unit for speed testing.

Good to know that in theory I can get 300Mbps over wireless if things are working fine - reassuring that there would be little point buying a 1Gb fibre line if the local network (mostly wireless) wouldn't cope with it anyway!

  @larkim if your connected to the main Deco via wireless then you should get much better throughput.

Basically there is no restansmission so getting 550-600mbps is not out of the question.

Mind you there is also the M4 itself and how capable it is in processing the throughput, I don't have a feel for that.

The fewer hops between the node and the Main Deco the better, every additional hop has to retransmit the signal onto the next device so throughput is halved on each hop.

Although it's not quite that simple because the signal will degrade with distance so the sync. rate will be less. The improved sync. rate due to stronger signal from connecting to a closer node can make up for some of the retransmission cost but usually not enough to make it worth while.

 So in principle a daisy-chained set of M4 units would see the speed halved with every step in theory?

  @larkim It does make you wonder when you think about it but, yes, since the network packets need to be received and then retransmitted at each node.