1 Wireless client transfers files fast, the other....very slow.

Okay, so this is a weird one and I'm inclined to think that it's not the network. But doesn't everyone blame the network first and then later find out the real problem?

I've got two wireless clients connected to the same AP at the same rate (174Mbps). The fast client can pull down a specific 500 MB file (Quickbooks file) within 1-2 minutes, the other slow client takes 15-20 minutes to pull down the exact same file.

The file normally resides on a QNAP NAS that both machines have access to.

Both clients have comparable hardware. Both have SSDs with plenty of space. Both have similar processors (Core i5 clocked at 1 Ghz). Both machines are running Windows 10 Home.

McAfee Antivirus is disabled on both machine (Windows Defender is still enabled).

The slow machine has almost no other software installed besides QuickBooks Pro.

Any ideas on what the heck could be causing the slow machine to be so dang slow?

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EDIT: Formatting. Also, both clients are in the same office within 15 ft. of the AP.