It looks like you're saving to a file in the same loop as you're doing the readings in. If you disable the file saving action, does that speed it up? I know you're only writing a small amount of data, but it could be slowing it down somewhat.
It also looks like you're using the drivers from the manufacturer. I don't have drivers for that exact bit of equipment, but I have used other Keithley drivers before. Those are usually good, in that they are functional, but they often have unnecessary extra functions in them. I would bet that if you opened up the "Read" VI that's in your loop, there would be at least one extra subVI in there (an error check), maybe more. If you can get rid of the error check that could also speed it up.
The last thing is another question. Do you need to see the data live, or do you just need to collect it at a certain rate and review it later? If it's the second one, look into doing a buffered read, where the equipment stores all the data on it and then you get it later all at once. I don't know if it's possible for your exact circumstances, but I know that in general it's possible. Someone about a year ago got it to work for their application:
https://forums.ni.com/t5/Instrument-Control-GPIB-Serial/Help-reading-buffer-in-Keithley-Electrometer...
