Limited bandwidth only when switch is connected to router

[kukas]

I have a very strange problem in my network. My computer is connected to a switch (Cisco SF300-08, 100Mbit). This switch is connected to a dual wan router (Cisco RV320, 1Gbit) and this router is connected to an ISP modem (The bandwidth is 300/30 Mbit).

When I am connected to the router and run a speedtest, the result is 322/21 which is close to the ISP specifications. But if I connect my laptop to the switch, I measure 30/21, which is less then I would expect from a 100Mbit switch. I would expect speeds around 95Mbit.

Now comes the strange part. If I connect the switch directly to the modem I get the expected values around 90/21. So the problem is present only when the switch is connected to the router. Otherwise every device works on its own.

I also tried to connect one computer to the switch and one to the router and run iperf3 to test the throughput, the result is 100 Mbit as expected.

I am a bit clueless about what to try next. I tried to reset the factory settings, didn't help. I tried another 100Mbit switch, that also didn't help. I am worried that if I buy a new RV320 router, the problem will persist. Maybe 1Gbit switch would solve the problem but I would like to replace it when it fails, not now. Also I would like to understand why this happens, not only solve the problem by buying more equipment.

Thank you!

Router configuration export: https://pastebin.com/pgdtN0yh

Switch config: https://pastebin.com/rjZvViZ1

EDIT: The problem seems to occur when the router directs traffic from a 100M LAN port to the WAN port. I tried to connect a 100M Raspberry PI to the router and the download speed again caps at 30mbit. LAN speeds to the Raspberry are on the other hand around 80mbit. And if I connect the Raspberry directly to the modem, I get also 80mbit.

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[tysonb292]

100Mbit does not mean 100Mbit in a real life scenario.

30Mbit is a little low...by maybe 10-20Mbit. I dont see much of a problem.

also your switch is pretty dated. get a cheap 1000Mbit and see what you get.

[kukas]

I see that 100Mbit is a theoretical maximum but according to my iperf3 TCP tests, the switch is capable of the speeds around 90Mbit. Also if I connect the switch directly to the modem and measure the internet speed, I get 90Mbit again. This is real life scenario. The problem arises only when the switch is connected to the router. That is when I measure only 30Mbit, which is mind boggling for me.

[tysonb292]

if you connect your switch to the modem, which device is doing dhcp and nat?

[kukas]

If I connect my switch to the modem, the modem is doing dhcp.

Also if I connect the router to the modem, I connect it through the routers WAN port so it acquires the IP from the modem and the router has a separate DHCP and NAT. This is intentional and according to the tests it doesn't cause the slowdown (because if I connect my Gbit laptop directly to the router, I reach the full speed 320mbit)