PCI-6251: speeding up analogue input and output

I want to use Matlab to try out a real time control idea. I've found an old National Instruments data acquisition card PCI-6251 which connects directly to the PCI bus (no USB latency) so I expected it to run quite fast.

However, running in a loop:

tic;
for i=1:1000
   vIn=s.inputSingleScan;
   vOut=2*vIn;
   outputSingleScan(s,vOut); 
end
toc

takes over 3 seconds to run, (i.e. 3ms per cycle). Any suggestions on how to make it run faster?

Alternatively, would it run quicker using digital I/O. I would not mind using external ADC/DAC to speed things up.