Per the IRS:
"As of September 25, 2021, we had 7.6 million unprocessed individual returns."
"Tax returns are opened and processed in the order received. As the return is processed, whether it was filed electronically or on paper, it may be delayed because it has a mistake including errors concerning the Recovery Rebate Credit, is missing information, or there is suspected identity theft or fraud. If we can fix it without contacting you, we will. If we need more information or need you to verify that it was you who sent the tax return, we will write you a letter. The resolution of these issues could take 90 to 120 days depending on how quickly and accurately you respond, and the ability of IRS staff trained and working under social distancing requirements to complete the processing of your return."