Encoding delay of Ethernet and the relation to cable frequency rating
If the delay down the copper wire is less than the encoding delay though, surely bits will be being received too quickly at recipient end, faster than they can be decoded into a digital stream which could be buffered?


I think the key point you're missing is that it's entirely possible for more than one bit to be "in flight" on the wire at any given time. 

For example, if the wire is 100 m long, and the velocity is 192 x 106 m/s, and the bit rate is 100 Mb/s, then 52 bits of data will actually be "on the wire" at any given time. The receiver, however will only be aware of the 1 bit that is actually arriving at the receiver at that instant.

If the transmitter is sending bits at 100 Mb/s, then the receiver must receive and decode these bits at 100 Mb/s. The length of the wire changes the latency time between these two events, but it has nothing to do with the rate at which the receiver must deal with the incoming data.

Usually the receiver doesn't deal with the incoming bits one at a time, doing calculations at 100,000,000 operations per second. Instead it simply queues the bits up into something like a shift register, and then operates on them at a much lower rate, maybe 12.5 million operations per second, but operating on full bytes with each operation (or even at slower rates, but operating on larger data words).