With the help and advice given on this thread I now have VLANS with bridge filtering. (Diagram below)
I have a few wired clients and when they download from the WAN at the maximum speed of my connection (110Mbps), my hAP AC cpu reaches 100%.
All the configuration so far is based upon this forums excellent VLAN guide.
There has been a suggestion that it may now be possible to optimise things further.

So is it possible to now implement hardware offload on the wired clients by using the switch chip, keeping the wireless config. as it is? Or am I barking up the wrong tree?

Now I know that this setup works (tested with hex RB as the AP) I plan to buy a hAP AC2 as the AP, so another option would be to use that as the main router and hAP AC as the AP. I am however keen to learn what configuration options are available.

With hAP ac, hardware offloading on switch chip can only handle traffic between devices in the same VLAN. If routing, rather than bridging, is necessary between the WAN link and the LAN devices (i.e. if WAN and LAN use different IP subnets), the only devices which support hardware offload of routing are currently some CRS models running ROS 7 beta.

On theory gAP ac has better wlan (3 chains for each band) than hAP ac2 (2 chains for each band). OTOH hAP ac2 has much better CPU. Which means that if you get a hAP ac2 you should use it as router and existing hAP ac as wireless AP, not the other way around.
Re. HW offloaded VLANs: @sindy explained it all.

Concur, another thought is that the RB450Gx4 routerboard has a similar CPU to the HAPAC2 but honking more RAM and memory which could also help (if you didnt need more wifi at all, otherwise the hapac2 as the router makes sense.)

MikroTik has too many SKUs. For 100mb service, consider the RB3011 or better the RB4011. Hang the Wifi AP's off available ports.

MikroTik has too many SKUs. For 100mb service, consider the RB3011 or better the RB4011. Hang the Wifi AP's off available ports.

Sure but does the RB3011 have wifi, I think he wants devices at both sites to provide wifi!

Thank you for the replies. I had not subscribed to my own topic so only just seen them !
I will check out the other hardware suggestions and thanks for the confirmation it is better to use hAP AC2 as router.

All my wired clients are indeed on the same VLAN so I think the switch chip option is still valid. This morning I have researched about using wifi together with the switch method and found this quite useful. I may try and setup this way just for the learning experience.

With hAP ac, hardware offloading on switch chip can only handle traffic between devices in the same VLAN. If routing, rather than bridging, is necessary between the WAN link and the LAN devices (i.e. if WAN and LAN use different IP subnets), the only devices which support hardware offload of routing are currently some CRS models running ROS 7 beta.

I have just tested with my old config with subnets on separate bridges and CPU usage is also pushing towards 100% when dowloading from WAN . I had not realised that.

EDIT: When transferring over Gbit LAN to LAN old config is 5% CPU and around 60% with VLANS setup - makes sense.

Sure, but does the RB3011 have wifi? I think he wants devices at both sites to provide wifi!

Sorry, yes the hAP AC2 would be better in his scenario.