They have step by step instructions in their documentation. They even give you the commands to run so you only have to copy and paste.
You literally git clone their repo, cd into the cloned directory, run a docker container and build the iso using the docker container. Took me 5-10 minutes using a single alder lake P core to make the .iso.
If pfsense can't do it I doublt opnsense can on the same hardware.
You can get pretty close if you throw more powerful hardware at it. I managed to route at least 1.4 million packets per second (~16.8Gb/s, 1500MTU concurrent upload and download summed together) using 4 alder lake P cores on pfsense+ 22.05.