Optimizing latency between application server (EC2) and RDS

HI, here's how the story goes.

We started transforming a monolith, single-machine, e-commerce application (Apache/PHP) to cloud infrastructure. Obviously, the application and the database (MySQL) were on the same machine.

We decided to move to AWS. And as the first step of transformation, we decided to split database and application. Hosting application on a c4.xlarge machine. And hosting database to RDS Aurora MySQL on a db.r5.large machine, with default options.

This setup performed good. Specially the database performance went up high.

Unfortunately, when the traffic spiked up, we started experience long response times. Looked like RDS, although being really fast for executing queries, wasn't returning results fast enough over the network to the EC2 machine.

So that was our conclusion after in-depth analysis of the setup including Apache/MySQL/PHP tuning parameters. The delayed response time was definitely due to the network latency between EC2 and RDS/Aurora machine, both machines being in the same region.

Before adding additional resources (ex: ElastiCache etc) we'd first like to look into any default configuration we can play around to solve this problem.

What do you think we missed there?