Hi Einar,
Being a fairly quiet night last night I was able to have a good read of the printer manual.
Two things stood out, no Linux drivers and no detailed specifications. I did manage to find the specifications on the Epson site where they mention 20 ppm single side and 10 ppm double side measured by the ISO standard. It is noted that these speeds are for windows drivers and that OSx does not have all the features and may not (read will not) get those speeds.
I then went on to look at the print technology used. It needs quite a lot of processor power and memory just to run the heads and that assumes that the PS and PCL emulation software supplies the page image in exactly the way the print head software expects to see it (this is why there are no Linux drivers and OSx has some problems).
This brings me back to the fact this printer is a sophisticated winprinter, while it will print postscript and PCL it is going to be very slow if anyone uses anything other than windows.
It appears that it is the win drivers that do the the multiple page preparation and send each page to the printer as soon as its buffer is empty. I have been, so far, unable to find a service manual for the printer which would usually give more information about the printer firmware and what the onboard processor actually does and where it stores all of its font information (HD???).
I don't think you are going to see very much of a speed increase, if any, by changing how it is connected to your computer. USB, WiFi, network cable will all transfer faster than the printer can handle what it considers as non standard input which it then has to convert for use by the print engine.