Last time I checked, mTLS incurred significant performance penalties and required significant soak testing to ensure that performance would be acceptable for a given application. If you're a small company, you have much lower hanging fruit to chase.
In my understanding there's additional overhead at handshake, but after that the performance is basically identical. The client certificate mostly acts to identify the client to the server, but otherwise the business of picking session keys etc is the same. At this point TLS overhead is close to free.
I think the start of this thread was a plea not to terminate HTTPS at the edge, but instead to plumb it all the way to the serving container. That's unlikely to be mTLS in any case.
