It's not Python that's slow, but scapy, which is dog slow. In fact, it is so slow that it should come with big WARNINGs that it's only really meant for interactive use. Do the dissection yourself or use something built for that purpose.

It's really surprising to me that lots of ppl are using scapy for things that require performance but then again if you look at the scapy website or the docs, it's not immediately apparent that their tool is not meant for this. Which I guess says a lot about the scapy developers rather than the scapy users.

tl;dr Scapy is a joke, performance-wise.