In the presence of certain polyols, a guar gum or similar polysaccharide thickener solution is boron crosslinked before achievement of complete hydration of the thickener, without compromising the viscosity level achieved in a fracturing fluid by the time it is pumped into the wellbore and fractures the subterranean formation adjacent the wellbore. Methods continuously involve hydrating a polysaccharide thickener to an extent of 10% to 75%, but less than full hydration. Before 75% hydration is exceeded, a boron crosslinker is added. Upon addition of the boron crosslinker, the fluid is injected into the wellbore to stimulate hydrocarbon production. Because less time is needed for hydration, well site mixing equipment is down-sized smaller to achieve better efficiency and cost savings.