Adding one gigabit swith slows download speed 50%

I have the following system:

ATT two pair wired service rated for 50 MIPS.

ATT BGW210-700 gateway

100' CAT 6 cable

Download is 55 MIPS at 42 ms

If I add a Netgear GS108 and 6' CAT 6 cable, Download is 27 MIPS at 57 MS.

What options do I have?

Is there a gateway with more ports?

Is there a better switch to use with the BGW210-700?

Talking to ATT Service and Service Tech's has been pointless.   I just upgraded wire from CAT5 to CAT6 to be 1GB complaint but no improvement.

Thanks