Will the airplane's cruise speed or top speed be slower with vortex generators?

No. We have flown a Baron with Dick Rutan at Edwards Air Force Base and used radar tracking to measure speed at different altitudes. The only conditions where we found a speed reduction was a 1 knot reduction in top speed at 12,000 ft. at full power, and no change in top speed at 75% power. There were no speed reductions at any other altitude. The small amount of drag created by the vortex generators is offset by a cleaner flow of air from the trailing edge of the wing.