Networking on host (in vBox application) is the root cause.
Change from NAT to NAT NETWORK or other.
Configure IP addresses accordingly.

Choosing anything other than NAT brings down CPU usage to 1% from 100% on host (in my case it was 25%, 1 core of 4 was running on 100%). Both Host & Guest now on 1-3% when not running any app.

This resolved the Host 100% CPU usage issue for me (Guest XPsp3 running on Host Ubuntu 24.04.3)

Deleted NAT entry in Networks in vBox application
Added NAT NETWORK, set ip to 10.0.2.0/24 (host)

Client end
10.0.2.4 - 253 (set ip to anything between)
255.255.255.0
10.0.2.1 (gateway)
DNS:
208.67.222.222 (opendns)
8.8.8.8 (google)