Performance decreased after Gigabit Upgrade

[PRW1]

I was using xfinity extreme (600 mbps download and 15 mbps upload) and my CM500 modem was chugging along just fine since it supports 680 download.  I ordered the gigabit service because it was on special and it would increase by upload to 35 mbps.  I don't need to gigabit download.  To my surprice everything reduced.  I'm now only getting 150 mbps download and 5 mpbs upload.  Is this normal?

[FURRYe38]

The CM500 is not reated for 1Gb ISP speeds, you need a CM1000 or CM1100 for that. 

What ISP do you have? I'm surprised your ISP let you upgrade with out checking your modem model.