Each tax return is evaluated on it's own merits. There is no first-in-first-out. Two similar tax return submitted at the same time can have totally different processing times based on IRS criteria for examining tax returns that the IRS does not divulge.
Some returns will be held for ID verification or other verification. The IRS will send a letter if they require more information to process your tax return.
https://www.irs.gov/individuals/refund-timing
Check your refund stratus here:
https://www.irs.gov/refunds