LAN with a slow router and a fast layer 2 switch: why does the router affect the ethernet speeds?

Question

Here's my LAN layout:

[Laptop (Windows 10)]  [Raspberry Pi (Ubuntu)]
                   \    /
               (Layer 2 Switch) 
                      |
     (Wireless router w/ gigabit LAN) 
                      |
     (Wireless router w/ 100 Mbps LAN)
                      |
                   Internet

I'm using the Raspberry Pi as a NAS, so I want gigabit speeds between the Pi and the Windows laptop.

The setup was LAN-to-WAN between the two routers, and I had gigabit speeds between Windows and Pi.

I've just changed it so the gigabit router is in LAN-to-LAN mode (ethernet plugged into LAN port, DHCP disabled). But now, Pi and Windows only talk at 338 Mbps! What's up with that?

My understanding is that the switch will direct the traffic directly from one to the other, without involving the routers. (Also, pathping/tracert don't show any other IP addresses between the two.) However:

1. When I run the iperf3, all three lights on the switch blink frantically - including the one to the router.
2. The speed is gigabit when I allow the gigabit router to be its own boss (DHCP enabled, connected WAN-to-LAN).
3. The speed is ~338 Mbps both when I set the gigabit router as the secondary router (DHCP off, LAN-to-LAN), and when I remove the gigabit router, connecting the switch directly to the 100 Mbps router. Same speed in both cases, so the slow router is clearly involved.

(Details: I'm measuring with iperf3. The Windows machine is using iperf3 in WSL, with an ethernet-to-USB 3.0 adapter. The switch is a D-Link DGS-105, the gigabit router is a TP-Link Archer C2, the 100 Mbps router is a Fritz!Box 7430. I've only measured with Windows as client and Pi as server; I can't get it to work the other way round, even with firewalls disabled.)

Answer 1

Problem resolved! I needed to do a DHCP release/renew. (Answering my own question so I can mark it as resolved.)

To any future readers: on Windows, that's ipconfig /release then ipconfig /renew, and on Linux, sudo dhclient -r eth0 then sudo dhclient eth0. (eth0 is the interface; check with ip a to identify the right name for your interface).

If anyone's got a few minutes to answer some remaining questions I have, I'd appreciate it! Trying to learn something from this.

Here's a timeline:

1. iperf3 runs at 338 Mbps and requires the FritzBox 100 Mbps router.
2. 12 hours pass. Unfortunately I didn't check iperf again before doing the following.
3. On Windows, I checked arp -a and the Pi was simply absent from the list.
4. I poked around a bit, including ssh'ing into the Pi from Windows (using hostname). When I checked arp -a again, the Pi had magically appeared! (I hadn't done a DHCP release/renew at this stage.)
5. iperf3 now yielded gigabit speed, but when I disconnected the switch from the router, the Pi and Windows couldn't ping each other. Also, the switch's port to the router blinked frantically while running iperf.
6. I did a DHCP release/renew on both Windows and Pi. Now, I get the same gigabit speed, but it works even when I disconnect the switch from the router. When connected during iperf, the port to the router still blinks, but less frantically than before.

What I understand from this is the following:

• When I was getting 338 Mbps, the switch needed the DHCP-enabled router for something, but probably not for carrying the traffic (because 338 Mbps > 100 Mbps). Was it perhaps querying the router to determine the destination of each and every packet?
• by SSH'ing from Windows into Pi, I somehow improved the situation but didn't give the switch full independence from the router(s). Perhaps this caused the gigabit router to learn how to answer the switch's queries, without involving the slower router?
• With a full DHCP release/renew, the switch became able to direct the traffic without a router. Was this because Windows and/or Pi learned each other's address, or did the DHCP release/renew also update the switch's routing tables (or whatever a switch's equivalent is)?
• Now that the switch can carry the traffic without the router, why does it still loop the router in (as indicated by the blinking light) if available?

Thanks for your time in answering any of these questions! And thanks to @user1686 for setting me on the right track.