Yep! I snagged it from another support thread, I don't deserve much credit.
I actually just subscribed to increase space in iCloud drive to put my big (100 GB+) iPhoto collection in there and keep it safe. Since copying it in, it's been chugging along for a while now (all day). In this time, I've seen the bird process running pretty high most of the time, dipping down to ~30% sometimes.
When I quit the process, I see the transactions stop, I see it re-establish itself, and pick back up (see screenshot).
Just below the screenshot, you'd see "typical transactions" start again.
And soon thereafter, I can see the bird process jump back up in CPU usage. I'm tempted to say that the process just ramps up and works hard when it has a lot to do, rather than it being a true problem. Albeit, it does slow other things down.
On the plus side, at least one guy tested and concluded it works quickly compared to competitors. And to note, other syncing services can have high CPU usage while they're syncing too.
My conclusion: if you put a big file in and you see the bird process ramp up, just keep calm and let it do its thing.