Establishing what makes a fast corner. Any drifting (entry, middle,exit) in a turn will produce a slower exit turn speed. On asphalt/concrete, using the maximum lateral grip available throughout the turn will always produce the best result exit turn speed. With braking and acceleration forces applied to the contact patch through a turn, total lateral grip is compromised by forward bite and braking force.
“Important” When a wheel is loaded Grip increases.
Keeping this as simple as possible
Load increases to the front axel on turn entry while under deceleration decreasing lateral grip on the rear. Lateral force overcomes lateral grip on entry, you lose the rear.
With Urethane there is a hard grip cutoff once lateral grip is lost . The lateral force must be decreased in order to regain lateral grip.
With rubber, lateral grip is lost on a curve that can be managed with throttle/brake control decreasing lateral force to regain total grip.
There are circumstances and design that have effect on lateral grip not discussed but these are the simple different characteristics between PU and Rubber.