why would a LAN transfer go slower when router is connected to network

Question

Before I ask, I realize the different hardware might be at play but I did some other tests that make me think its not the hardware and I wanted to ask to see if maybe there is something else going on.

I did a 1.5 TB transfer using this setup and it took almost two days which is roughly 10 megabytes per second.

• gigabit router connected to internet
• transfer is done using netcat
• source server (FreeNAS) connected via gigabit LAN: tar -cf - -C /path/to/data . | nc -N destination 9000
• destination laptop running MacOS connected via gigabit LAN: nc -l 9000 > data.tar
• router also has a Roku and Debian server (Intel NUC) connected to it but we hardly used the internet during the two days

Then I did a 1.3 TB transfer using this very similar setup and it took less than 12 hours. I didn't time it so I don't know exactly how long.

• different gigabit router not connected to internet
• transfer is done using netcat
• source server (FreeNAS) connected via gigabit LAN: tar -cf - -C /path/to/data . | nc -N destination 9000
• destination same laptop but with Windows 10 connected via gigabit LAN: nc -l -p 9000 > data.tar

At first I thought it might have something to do with the routers so I did a test to compare transfer speeds. I connected both computers to each router, with the router plugged into nothing else, and I did a speed test using:

• destination laptop running Windows 10: nc -v -l -p 9000 > NUL
• FreeNAS server: dd if=/dev/zero bs=1024K count=1024 | nc -v destination 9000

Both times I got about 100 megabytes per second. So I don't think the router is the problem. The only thing I can assume is that when I did the first transfer the router was connected to the internet. My wife does not appreciate this mystery so I cannot bring the servers/computers back down (where the internet connection comes in) to do a test which is why I thought I would try here.

If it matters, the router with the first transfer is a TP-Link Archer C7 and the second one is some propritary one my ISP provided that is locked down -- I can't access any details about it or change much.

Are there any obvious reasons the transfer speeds would be so much different when the device is connected to the internet? Does the internet aspect even matter or is there something else I am missing entirely?