Trying to do an initial backup to B2 over a consumer-grade cablemodem, I find that the default settings easily swamp my entire bandwidth. (FWIW, speedtest.net tells me I have 36Mbps down 6Mbps up when restic is not active.)
Even setting b2.connections=2 makes it fairly unusable. If I set it to just 1 connection, restic seems to hang indefinitely. It does not even get to the scanning phase. Is it possible that restic has a kind of deadlock situation when only a single connection is allowed?