That would have been nice to have it be that simple. I just did check the MTU and it is set at 1500. I'm using dd-wrt on a Linksys router but prior to that I had a SonicWall set up and had the same problems. I even had the SW help desk look at it but, of course, they had me bypass everything and just plug one pc into the sonicwall and into the modem and the speed was fine.
As I said I've changed every cable in this place to Cat 6 hoping that the larger pipeline would eliminate the issue. But it did not work.
I think I'm going to have to find someone local who knows what they are doing (surprisingly difficult to do) and can come to the business with higher end equipment or at least a good knowledge of Wireshark or similar and have them do some real testing. I have found, after spending a day with it, that wireshark is beyond my learning in any reasonable time frame.