Web socket vs Socket.IO speed?

I am trying to optimize speed sending video frames as base64 strings to an external server software.
The time difference between each received message in the server is measured and printed out for evaluation.
Using web socket plugins I never get under 33.5 ms time difference between each sent frame. I have tried three different plugins and they all show exactly this time difference on average.
Using Socket.IO on the other hand, I get an average of 10 ms.

Any ideas on why this is or what could be done to improve speeds? Multi threading, settings in the Editor etc?