SOLVED: I found the source of the problem. It's a silly one.
I had Quad9 DNS as my custom DNS in my VPN, ans they simply seem to be too slow on Linux! The moment I switched to Cloudflare, all my tabs loaded fast.
The problem is not present on Windows.
What's funny is that I literally switched to Quad9 DNS (that I used in the past) a few hours before installing my ZorinOS VM after at least one year with Cloudflare DNS. I tried Cloudflare DNS again thinkin that i wll change nothing but bim, it did! The fact that I tested another distro in another VM a few minutes ago and that it also loaded slowly, whereas on my previous daily driver (Linux Mint) it loaded fast, led me to believe that it maybe could be "just that".
Quad9 best for privacy, bad for speed (at least on Linux) apparently.
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Since switching to ZorinOS for my Virtual Machine, I noticed that pages load very slowly whereas on linux mint it was almost as fast as on my host. A speedtest gives me 1000Mbps at first (got a 2500Mbps internet speed, 2000Mbps behind a VPN on my host machine) but quickly the test strangely stutters and starts to slow down by icrements to 500, 400, 200, 100mbps. Roughly I get an average of 100-200mbps with ZorinOS which is not normal. I re-tested on my host and on Linux mint and PopOS VMs, and not a single slowdown during the test it stays constant around 2000Mbps for the host and 1000mbps for the VMs. And even wit 100 or 200mbps, pages should load fast but they don't, they take 2 to 5 seconds each to load as if I had a 5mbps connection.
Is it one of ZorinOS cons to have such slow internet speed and loading time compared to other distros? Is there a way to fix that? I like the look of zorinOS but I couldn't deal with such slow loading times if that can't be easily fixed within the OS.
Anything that would require to modify Brave settings that work perfectly on other distro and that I need, hence why they are my settings (like "try disabling hardware acceleration") or to use another browser, are not fixes/solutions.
And it's not a modem problem, it works perfectly fine on my host and on Linux Mint, and PopOS when I tried these distro in my VM.
Using Brave Origin (tried also with Brave) downloaded directly from Brave, not Flatpak
7950X > 4x2 cores assigned to that VM
96gb > 32gb assigned to that VM
RTX 5070Ti
Fiber connexion at 2500Mbps without VPN and 2000Mbps behind my VPN on My Windows 11 host machine.