Why is scp is very slow from local windows to ec2 c5.large instance?

There are multiple factors , the network bandwidth capabilities of your local machine also matter here , there will be additional latency over the public internet compared to a local network or Direct Connect . SCP can also be slow due to the overhead of the SSH protocol used by SCP. What is the purpose of this exercise or what is the expected outcome? This will help us give right solutions. A significant speed advantage can be gained by incorporating Amazon S3 into your migration process. You can get the benefit from S3's optimized data transfer capabilities. In order to bypass the SCP speed, you can upload your files to S3 and then transfer them to your EC2 instance. You can also try alternative methods like rsync (It have few optimizations to make the operation faster) Furthermore, what is the type of C5 instance you are using? Are you using the C5n, which provides high network bandwidth and I/O performance.

It's implied from your question that you are transferring data across the internet to the instance. The troubleshooting challenge here is that you (and no one individual) controls the internet. You also don't mention the latency that you have between the source and destination computers - that can have an effect on transfer speed as well.

To test the c5.large I'd suggest setting up another instance in the same VPC and see what transfer speeds you get. Assuming that you get acceptable times for the copy it will eliminate the instance as a potential source of the problem.