Slant routes are critically dependent on timing. The QB must throw to where the receiver is going to be before he gets there, and often before he even breaks. This is one of Fields’s glaring weaknesses - predictive throws that trust the route. I think one reason is that Fields gets the routes wrong, like he did when he threw the pick that was supposed to go to Kmet against the Broncos, and he doesn’t trust himself. Think about it - how many times can you remember Fields throwing passes while the receiver still has his back to the line, turning at the last moment as the ball meets him at the spot? This is the timing necessary at the NFL level. Screens, however, feature a stationary receiver. He hasn’t moved yet, or is moving directly toward or away, so there’s no prediction and less accuracy required.