26" wheels are relatively small diameter as they are. I don't like narrower than 1.75 on those wheels. If you want higher speed, get good quality slick(er) tyres, but I'd advise against going narrower. Definitely above 1.5", though I find 1.75 a sweet spot between weight (narrower are lighter) and (the pluses of wider tyres): larger wheel diameter, more comfortable ride, lower pressure (makes punctures less likely).