Originally posted by sinjamin:

its a video game, theres nothing realistic about it, surely a belt that gives the wearer super strength would also effect the cart aswell

No, it wouldn't.
https://www.merriam-webster.com/words-at-play/affect-vs-effect-usage-difference

Pedantry aside, you're confusing realism with physics.

It's not that there's nothing realistic about the game; we can wield fiery swords while fighting ice dragons, none of which is at all realistic... but there are rules.

There's a physics system in play; it operates by rules. Gravity presses downward at a specific rate, things weigh a certain amount, and those two facts alone mean that hauling a heavy load uphill is... well, slower and more difficult than hauling on a level field; the physics system says so.

First, let's get the weight units correct so we have an idea of what we're talking about; according to this post, it seems reasonable to assume that the unit of weight being used is the môrk; 216 grams, or just under a half-pound.

Therefore, assuming that Valheim is using merkur (the plural of môrk) as the weight units, the 300 merkur that a naked character can carry is about 65 kg, or 143 pounds. The Megingjord increases the carry weight by 150, bringing the total to 450 merkur (roughly 97 kg, or 214 pounds). Note: that's carry weight, not dead lift. You might be able to pick up a refrigerator, but you're certainly not going to sling it on your shoulder and take off running cross-country with it... and that's roughly the amount of weight we're talking about, here, and what our characters can do with it.

Magic belts aside, let's see how much weight is on this cart. There are 18 slots in a cart, each of which can hold a stack of "insert item here". Let's assume you've had a really good run through a few crypts and you have 10 stacks of Iron, already smelted up and ready for transport home.

Iron weighs 12 merkur per bar, so a stack of 30 bars weighs 360 merkur. This is already more than a character can carry without the Megingjord, but it's on the cart. There are 10 stacks, so we have a total of 3600 merkur... roughly 776 kg, or 1711 pounds. That's about the curb weight of a Mazda 121[en.wikipedia.org]... and that's not counting whatever the cart itself weighs. Another 220 kg (485 pounds) doesn't seem too unreasonable for something capable of supporting that much weight, so all in, we're looking at dragging a literal ton across muddy, rocky, hilly terrain (no asphalt or concrete in this fantasy realm).

If we were going for realism, I'd tell you to go drag a compact car around the block, never mind up even a slight incline, and tell us how fun and easy that was. For that matter, I challenge you to put a half-dozen concrete blocks in a child's Red Rider wagon, and drag that down to the pub or convenience store at the end of the block and back; then come back and tell us how you think you should be allowed to drag a literal one-ton cart up a grassy (at best) incline at a run... without breaking a sweat.