Gears not working quickly?
 

So basically my gears keep taking a couple of mins to change and make this weird crunching noise. Why is this? I’ve only had my bike for less than 3 weeks

So basically who knows with so little info, 3 weeks old, take it back to the shop you got it from, and get them to sort it, and tell you why.

Please provide more information.
Make - Model of bike
Link to the bike website
New or Used ?
Where did you buy it ?
Is the problem new, or happened for 3 weekes ?

Might be just a matter of adjustment that the bike shop will be happy to take care of. If it's a new bike, unless you are very competent and knowledgeable about everything about your bike, you should let the shop handle it so you don't jeopardize any warranty you may have had because someone told you banging it with a hammer is a good thing.