Software bridge overheads?

I am using OpenWRT in LXD, and it is working splendidly.
However, I was thinking of optimising this setup.
I have a 4-nic PCI card of which all ports are passed-through to LXD container of OpenWRT. I can use them as separate interfaces, or groups them together and connect to a bridge - which is what I am doing right now, because the setup is much easier (setting up only the bridge, instead of every interface separate).

But I wonder are there any overheads related to using such software bridge vs interfaces directly? The processor is AMD GX-420CA SOC.