Definitely weird. WiFi connections are poor, but VPN connection over the same WiFi link is good.
That makes me think perhaps DNS like others have said, or it could be something with your local routing table trying to reach something that’s not reachable. I would probably check the routing table first for anything weird. Like, you might have a static route applied from DHCP, but it’s ignored by your phone OS.
Since you can reproduce it in the browser I would probably look next at browser dev tools (F12). Go to the network tab. Then reproduce the problem. Once the task properly finishes, hit pause and sort by duration. You can also right click the headers and add a Timings>Latency column. See if there’s anything interesting.
Like, are the slow steps hitting a new domain name? Is there a slow POST among faster GETs? Is a step repeating after a timeout?
If nothing’s obvious there I would be tempted to repeat but lower level with wireshark to get the whole network picture. Get a good capture of the problem with a general sense of the timing of the problem pauses (in seconds from the start of capture). Find them in the cap and see what’s what. Compare good vs bad if no clear trends present themselves.
I was going to recommend a couple of Mikrotik 5Ghz bridge devices, but for the same money this would probably be easier to set up.
I don’t understand why Mikrotik doesn’t make a 5Ghz Wireless Wire kit. Their 60Ghz WW is great, and already paired like this TP-Link kit. But 60Ghz is so sensitive to line of sight interference.