Weird portal in maps since act 6

Hi there. This is probably obvious for the veterans but i'm new to this game. I'm now in act 6 and weird portals started to show on maps without any explanation. Here's a screenshot where you can see the portal and it's minimap icon.

I don't really know what that is or what i need to do whit it. When i "walk" through that thing i stay in the same map but the world turns "whiteish" and i think new types of monsters appear but other than that, i'm not sure. After some apparently random time (sometimes short, sometimes long), the map reverts to it's normal colors and sometimes i get some trinket out of the blue so i guess there's something special to do during that random time but what? Do i just have to run around randomly hoping to find the special monsters or is there something specific i need to do to get the rewards.
At least, the rituals are easy to grasp. Click the altar, kill the mobs, profit but that thing (is it from a previous league?) is rather obscure.

Delirium mirror. run through it. Kill stuff, quickly. Get a loot drop at the end of the effect. You'll notice the loot icons showing up to tell you what loot you'll get, and see them charge up as you kill stuff.

And yes... I agree with the posts after me that you'll take a performance hit while in delirium mode.

When you activate those portals it drops your fps down to 10. It's a special feature that ggg created.

that is path of exile's benchmarking tool. run through to activate it and see how good your computer is

Yeah you've got it, you'll get more out of it later when your character is stronger and you are in maps but for now just realize its a 'soft' timer where the fog is moving towards the end of the zone and you need to keep moving to stay inside it and keep counting kills towards the reward bubbles that appear in the bottom corner of your screen.

OK, thanks everyone.
Now that i how it's called i can look the wiki for more info but it's hard to search for something when you don't know what you're searching for.
...
Kind of fitting for "delirium" actually.

Best to beware of the performance hit. It can kill you faster than a boss can. Make sure you have your stuff up and running before you trigger the mirror.