How to analyze slow network speed on Macs

Question

I have 4 machines all on the same network (600mbps/60mpbs ISP link):

• M2 Mac Mini - ethernet, sometimes a page needs ~5 seconds to even start loading, when Slack loads it works but I can wait 30 seconds for a pasted image to load
• M1 Mac Pro - Wifi, same as above
• Linux PC - ethernet, completely fine
• Lenovo Laptop (windows) - WiFi, completely fine

What can I do to analyze the cause and improve network speed for the Macs?

EDIT: My Analysis

1. The difference of behavior between the Apple products and Linux/Windows machines is so big it's visible to a naked eye - I feel it's mostly about how a connection is established - Macs can take up to 30 seconds to start pulling assets, for the others it's always immediate.
2. It's an ongoing problem, does not happen 100% of the time but it's a day-to-day problem.
3. There does not seem to be a rule to what page is being accessed - I might have Slack itself loaded but a pasted image (which is being loaded from outside of Slack) takes 30 seconds to start loading, same happens to the likes of YouTube, comments, suggested videos on the right load but the video itself takes a moment to load.
4. Pings are always fine (~20ms)
5. I would make the guess that I'm capped at some connection limit, i.e. the system has 100 active connections and is not allowing to make more - something like that rather than bandwidth

EDIT 2: Further Analysis

1. Disabled IPv6 completely per comments - did not help at all
2. When a browser (Chrome, Firefox) is really stuck, it's stuck on "establishing connection".
3. Changing networks seems to help, please see image, using the LTE backup ISP solves the problem (which is not a solution but an indication of the problem).

Here's a summary of my network, red lines indicate troublesome connections, all IPv4, same DNS (8.8.8.8), green ones work great, although I'm still testing the M2 Mini to the backup ISP, but it looks like Macs don't work well in a Ubiquity network - which I heard about in regards to the Mesh WiFi networks but why would an Ethernet connection going through a simple switch and a basic router behave so badly?

EDIT 3: I was able to capture the network connection, this was Chrome trying to connect to Slack - 10 seconds "stalled":

EDIT 4: Firefox works great, no stalling at all.

Answer 1

The screenshot from Chrome showing that the time is spent in the state "Stalled" indicates that the browser is itself delaying the connection from happening. I.e. it not so that the browser has sent out some packet and is waiting for a reply from the server.

This usually happens when the browsers runs out of connections. It can do so fairly quickly as Chrome (and other browsers) have built-in, arbitrary limits on the number of connections they will hold for a server. This could indicate that your local network (i.e. router, firewall or similar appliance) or ISP has a broken network setup in terms of connection management.

You can test out these commands to change keep-alive parameters in order to ensure connections are closed out faster:

sudo sysctl -w net.inet.tcp.keepidle=15000 
sudo sysctl -w net.inet.tcp.keepinit=15000 
sudo sysctl -w net.inet.tcp.keepintvl=10000 
sudo sysctl -w net.inet.tcp.keepcnt=3 
sudo sysctl -w net.inet.tcp.always_keepalive=1 

This means that the system will use slightly more bandwidth on connection management, but will be able to discover broken connections quicker.