I'm trying to setup VPN for my own use. I have noticed very strange issue with speeds which I struggle to understand.

Question: What is causing slow download speed from AWSFrankfurt when using VPN?

[HOME] My ISP home connection with bandwidth 30mbps
[VPN]: AWS-Singapore, t2.medium, Ubuntu, l2tp ipsec.
[AWSSingapore]: AWS-Singapore, t2.medium, Ubuntu
[LinodeSingapore]: 20$mo ssd node, Ubuntu
[AWSFrankfurt]: AWS-Frankfurt, t2.medium, Ubuntu

Download/Upload in mbps
1) HOME <=> AWSSingapore 30mbps/30mbps
2) VPN <=> AWSSingapore 600mbps/600mbps
3) HOME <=> VPN <=> AWSSingapore 30mbps/30mbps

4) HOME <=> LinodeSingapore 0.1mbps/15mbps
5) VPN <=> LinodeSingapore 30mbps/300mbps
6) HOME <=> VPN <=> LinodeSingapore 20mbps/10mbps

7) HOME <=> AWSFrankfurt 2mbps/30mbps
8) VPN <=> AWSFrankfurt 60mbps/40mbps
9) HOME <=> VPN <=> AWSFrankfurt 5mbps/20mbps

I started investigation because of line #4. I could barely connect with SSH. Then VPN solved that issue for me. But not on line #9.

How is that possible?

I'm using l2tp/ipsec only to connect HOME<=> VPN all other connections are plain\open connections.
I have root access to all severs above
All nodes have nearly zero CPU usage during tests.
I'm testing connection speed using iperf -s and iperf -c IP and downloading file using curl http://.... from nginx server.
Results are the consistent over time (multiple days).

Line #3 means there is no problem for VPN node to handle that much traffic in terms of CPU.

Question: What is causing slow download speed from AWSFrankfurt when using VPN?

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