Hydraulic lifters are subject to this thing called "hydraulic intensity" which limits how quickly the cam can open the valve without "collapsing" the lifter mechanism (which would be detrimental in terms of performance). With a solid lifter, the cam profile can be more aggressive and can accelerate the valve off the seat and open it fully more quickly, resulting in "more area under the curve" (more valve opening during the same period) than is possible with a hydraulic. Because of the difference in ramp rates on the camshafts, in general it's not wise to put a solid lifters cam on hydraulic lifters, and vice versa.