(Somewhat related to Do different star systems experience time differently?)
In my futuristic settings1, Humanity discovered a network of portals allowing for instantaneous travel (hand-waved, their exact nature is irrelevant to this question, but these portals are miraculously exempt from science).
I want to have such a portal leading to a survivable2 planet with this special quality - time there run much faster than Earth (or anywhere else in the Solar system, for that matter), ideally, every 10 days "here" takes around 40 years "there".
Is there any set of conditions that can create such time-differential effect?
1: This is intended for a pen and paper role-playing game. Players will hear description of the setting. Their characters will start on that planet, the goal being to keep the players ignorant of their exotic state for the first chapter of the campaign.
2: To clarify, "survivable" in this context means that two-three "generations" of a tribe of "cavemen" planted there by advanced humans could survive (so around 80-120 years in their local time). These cavemen may be engineered/augmented if necessary, but only in ways that will not be immediately obvious to inquisitive players. They are transported there after any preparations already took place, including creation of a working ecosystem. The planet should have a sun, breathable atmosphere, gravity and other conditions similar to Earth's. Other than that any exotic conditions which make this time-differential scientifically possible are fair game (orbiting a black hole, rogue star system traveling at near light-speed compared to Sol or the Milky Way, exotic material composition for the entire system etc. etc.)
Bonus points if you can mention any problems your set of conditions raises (e.g. traveling at near light speed means continuous bombardment by cosmic particles), and even better if you can suggest mitigations for them (e.g. have the planet orbit around a gas giant with a strong magnetic field that will divert most of the particles).