You have to set Polygon gas fees to Aggressive because MetaMask's suggested fee for the Polygon network is usually too low.
This means that your transaction can take a long time to process if you're using MetaMask's suggested fee.
Polygon has very low fees - this is one of the reasons it works well as a network for NFTs.
However, one of the consequences of these low fees is that the network is often congested, or busy with lots of other transactions.
In order to avoid delays, set your Polygon gas fee to Aggressive. Your transaction will confirm quickly, and the fee will still be quite low!
Don't do this when you're using the Ethereum network - you'll pay too high a gas fee.
How to set your gas fee to Aggressive on MetaMask when using the Polygon network
1. When your wallet opens to confirm a transaction, click on Site suggested.
2. Then, click on Aggressive.
3. Finally, double-check the gas fee and click Confirm.