In a cavalry charge, the shield protects as much (or as little) as otherwise. The horse is doing the actual work, while the rider can use the shield normally.
An infantry charge starts at the last moment. At that point, the infantry formation has been in the effective bow range for a couple of minutes. It would be stupid to exhaust yourself by running a long distance over rough terrain carrying a heavy load before the fighting even starts. And shields tend to be large enough that the additional protection from them outweighs the gains from reaching the enemy faster.
An infantry charge starts at the last moment. At that point, the infantry formation has been in the effective bow range for a couple of minutes. It would be stupid to exhaust yourself by running a long distance over rough terrain carrying a heavy load before the fighting even starts. And shields tend to be large enough that the additional protection from them outweighs the gains from reaching the enemy faster.