I am using an EC2 micro server with PHP and an RDS 2xlarge server with MySQL. During periods of high traffic, the system experiences slowdowns, but the CPU and memory usage on the EC2 instance are still within comfortable limits. The CPU and memory on the RDS instance are also very underutilized. However, the web server becomes extremely slow. I am using Nginx as the web server engine. I am not sure what the cause could be.
I have checked the CPU and memory on both the EC2 and RDS instances, as well as the disk I/O on the RDS, and there doesn’t seem to be any significant load.
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You appear to have selected a t2.micro or t3.micro. They are Burstable performance instances – Amazon Elastic Compute Cloud that have limits on sustained CPU.
They are great for bursty workloads, but if they try to use a lot of CPU for long periods, then the CPU will be limited. When you say that CPU doesn’t have a significant load, it might be because the instance is being forcibly throttled rather than not having much demand.
Try changing to a different (non-burstable) instance type, or active Unlimited mode on the instance (extra costs apply).