File processing much slower on SpringBoot app deployed via AppRunner compared to EC2

We are currently experiencing performance issues within our SpringBoot application which is deployed as a docker image via AppRunner. Code under test uses Java library which does Zxing QR scan under the hood. Running the code locally, in unit in integrations tests, processing is fast as expected, somewhere between 500ms and 2s depending on the file. Dockerizing the application and deploying it, we can notice extreme numbers for processing in CloudWatch logs, somewhere between 5s and 160s for the same set of files. Tweaking AppRunner core and memory configuration didn't change a thing. After running out of troubleshooting ideas for code refactoring, we tried to deploy application on medium EC2 instance. Numbers started to make sense, processing being in range between 500ms to 4s. Any ideas what could be the reason? Are we missing some configuration for AppRunner?