Help for understand graphic difference HDD/SSD on FreeNAS

I am on FreeNAS and I replaced an old HDD by a SSD in a RAIDZ2 vdev.
(With time, I hope replace all old HDD by SSD.)
But in my drive graphics I see this :

Someone can explain me :
-what the delete peak mean ?.
-why more latency on SSD ?

Thank you in advance