RDS MySQL Metrics

Hello,
I'm setting up one Magento site just for testing performance
Among other things this Magento site using REDIS for session storage, and backend cache.

I have problem that when I open 10 links of that website, 5 are broken with error 503 "Service are unavailable"
I checked EC2 and did not found bottleneck, nothing was overloaded.
When I check RDS Metrics, CPU Utilization peaks are max 30%, but IOPS Read is up to 70 at that moment when I got those 503 errors.
RDS instance is t3.medium.
I don't understand that IOPS metric. What is maximum for this instance?
If IOPS is problem, can I somehow avoid reading and writing on SSD, with keeping everything in memory? Database size is 2GB at all