There is no performance difference between QODBC Server Edition license and QODBC Read/Write license.
There is no difference in the commands, queries, data or schema between QODBC Server Edition license and QODBC Read/Write license.
QODBC is a client-side driver that would normally reside on the workstations/client machine(s).
QODBC Server edition license is required when you have any service-based application like SQL Server, Web Server and Windows Service connecting to QODBC or if you are installing QODBC on Terminal Service/Remote Desktop Service enabled machine.
QODBC license depends on the application you want to use. When the machine using the services likes a web server, PHP MS SQL Server, windows task scheduler and connecting to QRemote/QODBC need a server license.
So If you are using any service-based application, then you need QODBC Server Edition.
If you want to use a desktop based application like MS Access, MS Excel and you want to Read Only access, then you need QODBC Read Only license.
If you want to use a desktop based application like MS Access, MS Excel and you want to Read Write access, then you need QODBC Read Write license.
QODBC licensing is machine wise. If you have QODBC Server Edition license activated on your Server, your workstations if using QODBC/QRemote driver(s) directly will still need a QODBC License. You need to buy a license of QODBC/QRemote for each of your workstations in which you want to use QODBC/QRemote driver(s).
QODBC Licensing Information
QODBC with QRemote Licensing Information