OK...so if I am drawing the picture of this setup correctly...you have 2x computers. They are connected to each other using a patch cable from NIC to NIC. Assuming the patch cable is CAT5e minimum..and properly put together...factory made? Yes, exactly right
One of the PCs has a USB cellular modem...which should be irrelevant of the two computers connecting together so we'll leave that alone. yup
You should not need a switch in between the computers...they're just doing a direct connection to each other. Part of the gigabit standard is auto MDI-X...so a crossover cable is not needed either...the NICs do it themselves, just a standard CAT5e patch cable is all you need.
Are the computers set on static LAN IP addresses? no, does it matter with only 2 pcs?
Is netbios over IP enabled in the advanced TCP/IP settings on each? no
Firewalls rules set on both for workgroup file 'n print sharing? I think so, I'll have to check it again
Any 3rd party software firewalls/security suites installed which is bloating things up? no, in fact the client pc has no antivirus at all since it does not go on the net.
Speaking of antivirus....set exclusions in the real time scanner on the host PC for the directory which is shared. good idea. And on the client PC...set exclusions on network drives/folders...including this folder it's reading on the "server".
What network cards? Realtek RTL8103EL Intels are best..Broadcoms OK, I don't like realtecs...but for only 2 computers it shouldn't matter much...it's not like you're trying to serve up files to dozens of clients.
Have tried turning on jumbo frames on each computers NIC? Try setting to 9,000.I will check on this! don't know if the cards have the option. thanks great info!