Network transmit queue limits

It is worth keeping in mind that slow start is not very slow - it is a doubling of the congestion window (and hence average transmit bandwidth) every round trip time. If you don't have something like slow start a new connection tends to immediately saturate every bottleneck link, causing large scale packet loss not only on the new connection, but all the others using the link as well.

That puts all the (congestion controlled) flows on the link into some sort of recovery mode, which is generally much slower than slow start is in the first place - a constant increase every RTT rather than a multiplicative one.

It works, the flows do sort themselves out, but it isn't very friendly, and usually doesn't even help the new connection. That is why they adopted "slow" start in the first place. It replaced previous practice of saturating the outbound link until some sort of loss indication was received. Running a gigabit per second flow into a ten megabit per second link doesn't work all that well.