First of all, you need to localize the root cause:

1. Does the download speed(or throughput) change over time and is lower than usual for the problematic sites/pages? Try to download test data samples in parallel from the other domain. And add Mbit/s metric to monitor the target sites. It will help you to find out if the target sites limiting you or not. If so - try to spread the data flow among unique IPs. The basic principles can be found here datascrape.tech/blog/pyramid-of-efficient-scraping

2.Double-check the pages/URLs queue size. It might be that you have local maximums of URLs number and this causes the problem, not the network throughput.

3. Double-check that you haven't reached your VPCs throughput maximum. Haven't checked that recently, but usually throughput(network speed) depends on the disk size and is limited by default for the VPCs.