Just looking at it visually it seems Orion is (perhaps?) taking a more roundabout (slower) route, and it's also ending up at the moon in a different orbital configuration than Apollo.

Another factor, I am sure, is cost. The faster you want to get there, the more fuel you need to burn. I'm also going to guess it's not a linear equation, which means the faster you want to get there, the fuel requirements will increase in something like an exponential proportion.

Therefore slower is cheaper, to a point. If you go TOO slow, your astronauts will starve, or you need to bring more food and provisions, which will cancel the cost savings on speed reduction. So somewhere in there is going to be an optimal cost/fuel/food/provision trajectory for each mission.

So, in summary: different mission, different parameters for optimal execution.

Artemis I launched at 1:47am EST on Nov 16 and entered lunar orbit on Nov 20th at 2:09pm ET so roughly 4.5 days. Remember too that Artemis I isn't meant for a landing, instead it entered a distant retrograde orbit which orbits at a considerably higher distance from the surface. Spacecraft move considerably slower at higher apogees, so I don't think there's anything suspect about Artemis I taking a bit longer to reach it's planned orbit around the Moon.