If you're running on a server with 1Gbps, then yes - it can be an issue. Another issue (I'm going to add a section on it) is that you can peg the CPU at 100% very easily. Parsing HTML / Running xPaths uses a lot of CPU, so if you're running a good number of threads this can quickly become a problem.

It's an issue if you have high internet speed. I've had cases where the data can be downloaded faster than it can be parsed and overflow the internal queue. It's even worse if your crawler is not asynchronous, in that case slow parser slow down crawling speed as well.