umm yeah, printing faster than 150mms on an i3 is probably more than a little ambitious.

to get prints that fast on any machine, you need really good higher-temp pla and - obviously - change the print head temperature.

It also depends on the layer height, nozzle diameter etc.

But basically you don't have a problem, just unrealistic ideas of how fast you can print :-)

Can't help with cura, personally i think it's the worst slicer out there.