It doesn't - if done right.
When hibernating my laptop it writes the memory contents into swap - which is also encrypted. Yes, de-hibernating is slower if contents need to be read from disk. OTOH it's still faster than booting from disk.
When hibernating my laptop it writes the memory contents into swap - which is also encrypted. Yes, de-hibernating is slower if contents need to be read from disk. OTOH it's still faster than booting from disk.