fearsomepirate
Hero
I have a player who plays a 7th level wizard and a 7th level cleric at the same time. He has spell cards for each character. One stack for attack spells, one stack for defensive spells and one stack for utility spells. He plays faster than the fighter who only has a single and a simpler character. It's a question of personal investment and player ability.
No matter how fast he is, in three sessions, the wizard and the cleric are just not gonna cast that many fourth-level spells. If you're playing at the WotC recommended speed, the party has one or two more sessions to grok how the dynamics have changed before they change again. Three sessions later, fifth level spells come online. My point is that 5e is a game of constantly-shifting PC abilities, not that there aren't players who have practically memorized the spell book.