SSD is good.
Habe you tried to set the WEB_CONCURRENCY?
Here are the performance documentation about it: Configuration via Environment Variables — Zammad System Documentation documentation
We have 60 agents at the same time on our instance with 140.000 tickets (most are closed) and no problems after adjust the parameters.
Our setup: 6 CPU cores, 16 GB RAM on SSD Storage
MrGeneration described it here too: Performance Issues with 50 - 60 concurrent users - #2 by MrGeneration