I've ordered my first firearm since moving to IL and I'm a little confused on the firearm purchase process in the Land of Corruption. In most other states (when doing a transfer), your local FFL receives your firearm from USPS/UPS/FedEx, then you drive over there, fill out the 4473, he submits the NICS check, you get an approval, and finally you drive away with the firearm in hand.

The process has been mostly the same in IL. The waiting period isn't a big deal because it will take a few days for the shipment to arrive. So far, so good...

My FFL sent a copy of his FFL to the seller, the seller shipped the firearm, and my local FFL received the firearm today. Easy enough. However, here's where the weirdness starts... My FFL received my firearm today, so I was expecting to be able to drive over there and pick it up today. NOPE! Apparently my background check is still pending and will likely be approved on Monday.

In IL, does the FFL submit background check information before the 4473 is even filled out? Will an additional NICS check be required when I go to pick it up? What's the point in having a background check if you already have a FOID (where a background check was done and is likely regularly checked)? Is this normal for background checks to take days?

My FFLs, have always called in after the 4473 was filled out, (which could be done before the gun is even shipped).  If you do it that way, the check should be done, by the time your firearm comes in, unless it's a handgun, and the seller ships it fast.
Some dealers, don't want you to fill out the 4473, until the gun comes in, if it is used, and may be sent back to the seller by you.

Illinois.  Checks are always backlogged I always get delayed .

How long does it take for an approval?

It can take several days depending on how  backlogged they are , similarity in names/birthdates etc.   One misconception is they have to release after 3 days but not so.  In Illinois, once the process is started, the FFLs must wait for a response.

Some  people always seem to get delayed and others  never do.

When I lived in MO, I got "further review" 99% of the time. In Texas, it was instant approval. Since my name is fairly common, I'm guessing that's why.

I've never seen one run a background before the 4473 since they get charged for the background check ($2-$3).

Illinois implemented an online system for dealers a while back, this has greatly improved the system and most of the time they get an instant authorization.

Dealers can technically release the firearm after the waiting period if the check has been delayed, most will not.

https://ispffl.com/Public/FAQ.aspx

What happens if a customer orders a firearm, and when he comes into pick it up, ISP issues something other than an approval on the FTIP?

The actual transfer of the firearm cannot take place until there is an approval from the FTIP system, regardless of when the agreement was reached. If the FFL receives a transaction number, they cannot complete the transfer until they receive an approval.

This is my first firearm purchase in IL. I wonder if that's why the check is delayed.

If your name is common, like William Jones or Mark Davis, it may take a bit longer for the approval to be issued if there was someone with a similar name and DOB in Illinois or your previous hometown who had legal issues.  

If you have ever had a legal issue that could have resulted in a criminal conviction, it could delay the process while records are being checked.

I picked up a firearm at a FFL last week and the approval took about 15 seconds.

What ffl did you use?

Could be wrong but I thought that Illinois plays by the ATF's rules.   ATF I believe states that if there is no response from the state, they can release after three days.   At least that's how my FFL explained it, however, he won't release without the states approval as well.

My FFL submitted the background check on Friday (the day my firearm arrived). It was approved on Monday. When I picked it up, I just had to fill out the 4473 and pay the transfer fee.