Depending on how old the 15" iMac is it may only have USB 1.0. If so, there won't be any speed improvement going to a USB adaptor.
You can look at using an ethernet bridge. An example would be
this
You would connect to it with your ethernet port and configure it with a browser. There are other brands out there too. Make sure they can be configured with a browser since most only come with PC configuration apps.
I have not used this unit so I can't comment on the ease of use.
Scott