Multiplayer connection problem

[Dokkodo]

I've been playing factorio off and online since it opened and in the earlier days I got many connection issues but after patches not so much.
Recently I've encountered a problem.

My brother and I are on the same ISP and he gets "Unable to establish network..." error when trying to connect via public browse, nobody else has that problem connecting to me.
When we try to connect via Hamachi, it connects (through direct connection) but downloads at only a handful of kbps. It takes 90 minutes to download my AB map and then disconnects. One time he disconnected and got a message something like "[player name]'s computer could not keep up with server". He's playing on a macbook, but has never gotten this error before recently, even when we had a massive AB megabase easily 4 times the size of our current map.

Please advise which details I can provide the lovely people of this forum to try get to the bottom of this.
TIA