Strange behavior on wifi network

My girlfriend is complaining a lot about wifi slow issue. The connection is a solid 300mb, on a lot of place she has trouble loading picture. I run my pcs on wired LAN, and I feel OK. My phone is android and it is feeling OK too. Her phone is a iphone and I don’t know if they do anything different like bypassing DNS or I don’t know. Her old macbook air (2011) shows problem too even though the wifi g should be working for that.
I have tried using provider DNS, it is usually the worst. So I have tried the other proposed by Turris. They do work for me but I keep having complaints, so it is pretty hard to troubleshoot.
When I test the DNS the first one is fast and second one always last much more like 5 or 10seconds, is it normal behavior?
Any ideas?

Well I have spent some quality time with her phone… In settings wifi (I) you can access to dns. I have been tweaking the dns there. Removed automatic and then on one wifi there was the ip of the router (I have left it) one ipv6 removed and then I have added Google dns. On the other one I have added Google dns and now no more complaint. I have tested myself first. Huge improvement in vinted app for exemple.
So problems looks solved but any advice is welcome. Off course she can’t get rid of her iPhone don’t propose that…

This sounds really weird to me. I believe Androids (still) use DNS from the network by default (i.e. the one ran on Turris), so to me it’s suspicious that those apparently don’t experience problems but the iPhone and macbook do. (I’m not confident I fully understood all of the text, but this seems clear.)

Anyway, if DNS is a cause here, I can imagine two caveats. I believe the default Turris setting for the DNS server is to use ISP’s as upstream (with local caching, validation, etc.) That doesn’t sound good if you say the ISP’s DNS is “the worst”. So at first, I’d try tweaking that in (re)Foris DNS tab – either uncheck forwarding or choose some other upstream provider (I see you mentioned Google – they’re in the easy list as well).

The second issue I sometimes hear is if IPv6 towards internet doesn’t work; I think sometimes it’s “badly detected” and slowing down DNS due to not getting replies over IPv6 even though expecting them.

I think my ipV6 wan is not working (well). Removing my ISP modem to replace it by the Turris has been a huge work (and pain), but it does work fine with ipV4, ipV6 shows bad at the testing. I don’t know how to smooth that out neither where to start.
Iphone is working well now in manual.
Macbook I don’t know, I will check another day, we dont use it much lately.
Forwarding is currently ticked. Not sure to untick right now that it is working.
Sometimes (once a month?) the wifi gets stuck too at all devices and then it takes several reboots to get it back to work, but I have never fully understood where the problem lies, last time was when I have changed my provider up and down speed. Maybe trying to remove the forwarding might help…

So I did more test.

Removing the forwarding…
More or less the same.

Tweaking wifi on iPhone removing ipv6 adresses
Sometimes huge improvements. Sometimes not

Removing ipv6 on turris. I don’t know how to set it up properly on turris so I removed it.
After reboot did not notice improvement. But sometimes later huge improvement on iPhone.

Turning forwarding back on.
Sometimes I have issue on Android dns too true before and after.
MacBook air (I don’t test everyday) could not grab emails and it did on phone hot-spot wifi.
Still complaints from iPhone.

I am getting crazy here…