If you have a high ISP engine that is low thrust, your rocket will either not get of the ground or if it does get off the ground it will suffer from a very high

If you have a high ISP engine that is low thrust, your rocket will either not get of the ground or if it does get off the ground it will suffer from a very high amount of gravity loses. High ISP on the engine doesn't matter when your rocket spends a long time just fighting gravity. High thrust to weight ratio is important for a rocket. The higher your TWR the less gravity loses your rocket suffers. There is a reason most rockets main liftoff comes from lowers ISP RP-1 engines or solids rocket. High ISP hydrogen engines just result in losses. SpaceX Raptor actually deliberately reduced its ISP to increase its thrust.