Thank you for your reply.
As you suggested, I tried reverting back to “factory default-like” settings:
eth1 WAN, eth2-5 LAN with 5 being configured as slave with DHCP and NAT.
The good news is, using this config CPU usage dropped in half, peaking at ~30% with 60mbit of WAN to LAN traffic using the same speed test as in the first post.
I found this post stating a RB450G with NAT would max out at 200Mbit, which seems on par with the aforementioned results.
However, with this config I am unable to set up hotspot: the server item at IP->Hotspot in Winbox is now red colored and unable to proceed.
It looks like this has something to do with Switch Chip Features but being a novice Routerboard user, I don’t fully understand what the relationship between both setting a master port with slaves, hotspot and throughput performance is.