They will not hold your 2021 return because 2020 is still processing.  But if you know the issue with your 2020 return, you will want to make sure it is not repeated on the 2021 return.

Just wondering whether you know the reason why it takes so long for them to review your tax return? I will have to prepare tax for my company next year so just want to be prepared.

It's taking longer due to Covid-19.  And there are millions more returns to review this year.   More people are filing to claim the Stimulus payments.   Any return that has an amount on line 30 for the Recovery rebate credit is being reviewed.  

If your return wasn't approved and finalized in 21 days, it means the IRS found an issue that needs review by a human person.

Now you are in the "pipeline".

IRS latest news is that they have 9 million returns still to look at.

Based on what I see on the IRS procedures on the Internet, your return gets passed around from desk to desk until somebody marks it "Good To Go".