Wonder and hoping if anyone could help, this is weird.
I've got a Pi3 all set up and running as a VPN-AP, it is rock solid, phones, laptops, a firestick, PC etc all connect to it, all through the VPN (Airvpn) and the firestick barely buffers.

Then I try and add a Pi into the mix to eventually host a samba share on it but the Pi is garbage on the network, kbps only download speed and drops, kills ssh, lags etc etc. I thought this could maybe be a problematic Pi4 but I've just hooked up a brand new 3b+ with a clean install of raspian and a new SD and it is just the same. All that I've done is put ssh and wpa_supplicant configs on the SD.

Setup is Pi-AP to router on Eth0
Wlan0 is 4.1 and serving 4.2 to 4.20
Got hostapd dnsmasq and openvpn
Every device in Lan has static IP within the range, with 4.1 as gateway and dns (works perfectly on everything).

I've tried setting and/or not setting static IPs/routers/DNS in dhcpcd.conf on the client Pi3 and old Pi4, tried everything 🤔 Always weird and sometimes kills ssh, occasionally killing ssh reconnects and requiring power resets.

Only thing I can think of is it is something to do with local traffic/openvpn though it allows that by default I think?

Hope somebody can think of something else? Pls help 🤦*♂️

Just a wild guess, perhaps the hostnames are the same for both Raspberry Pis? If you haven't changed the default hostname in raspi-config then they will be fighting over the same name.

No, they both have separate hostnames. I've just tried again (while i'm watching perfect VPN tv on the firestick) and Speedtest-Cli gives me low latency and good speed but when I try to wget a testfile it hangs and drops the SSH, it is mental, I would assume the pi would talk to the pi better than anything else, especially as it is nearest.

Never assume anything though, as they say.

A quick search found various numbers of max wireless clients i.e. 8 16 or 32, how many total devices are connect to the access point?

A plain Pi3 wifi speed is only about 38 Mbps so you might of reached a limit on the number of devices and bandwidth limit by adding the Pi to the mix.

Version 3 Pis still shares bandwidth with the USB bus so if anything is anything connected to a USB port will affect bandwidth. You might be alright if you use the Pi4 or maybe the 3b+ as the access point instead of the just the 3.

What about temperatures? Are you using a heatsink? You might be running into thermal limits.

I thought exactly that as Air have a 5 device limit to an AP. I've been in touch with them and they have confirmed there has been no throttling of any device at their end.

My comments were just for the Pi3 as an access point and not the VPN server.

What happens if you disconnect everything but the Pi4 from the access point. Do you still have the same problems? If not add them back one at a time until you start having problems. What is that number of devices?

Ok thank you, I thought you meant the VPN. There are only 5/6 devices in total anyway, but the problem is there with only a laptop (ssh) and the problem Pi connected to the AP.

Just as an aside I would connect the Pi4 using ethernet to your router versus wifi if its intended purpose is for sharing files. WiFi bandwidth is divided between the number of devices connected to the access point. The 4 does not share bandwidth between USB and ethernet so it will have better performance.

Have you check the CPU temperatures on the Pi3 AP yet?

The "Purpose" of the whole OP and project is a Wi-Fi access point running on a Raspberry PI, serving clients via wifi through the VPN. I realise I could plug the Pi into the router though that would bypass the entire project and in that case this post would not even have existed.