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Actually, what would that be good for? As long as a relay is so lightly loaded that the active connections each can have more than than, there is no point in throttling them, and as soon as there isn't, they're fair-share-throttled down below that anyway.
Uhm, my thought was to make Tor connections "unattractive" to file downloaders, e.g., P2P, by limit connection speed via Tor. No-one using email via Tor needs a fast connection, well, unless they send lots of huge attachments.
I guess I was thinking along the lines of someone having to "assemble" their P2P download out of 100-1000s of slow connections (if all Tor connections) vs using 10 fast connections, thus discouraging such use. P2P would be slower, yet others using the same relay *for anonymity/privacy* would not be affected as much. I am a Tor newbie, so probably need to understand better how Tor mixes up connections via relays, etc.