I have just finished reading an insightful post from Zac Charles, where he has compared the performance of the different .NET Runtimes on AWS Lambda. His conclusion is simple:

If you’re making a new Lambda function, should you choose .NET Core 2.1 or .NET Core 3.1? You should choose .NET Core 3.1.

The piece is full of great insight and explains how performance has been improved in the 3.1 runtime.

If you want to read the post your self, check out the following link: https://medium.com/@zaccharles/a-close-look-at-net-core-3-1-on-aws-lambda-9ccec4dd96be