It's possible to lose speed through the equipment you have and the way you have it set up, but by half, I doubt it unless you have quite old equipment
How old is your modem?
How old is you PC?
On the modem front,you should search the manufacturers website to see if there are any new drivers more suitable for your new connection type as a first step.
Then it's down to how you configure it.
The newer modems are quite a bit better than ones made say, 3-4 years ago.
This is particually true of wireless ones.

As for the PC, baseline, that will decided by the ethernet connection speed of your PC's MB, something that you can't change as it's board components that decide it.
Then it's down to CPU type & speed, RAM amount & speed for loading the pages [try upping your virtual RAM as that can speed things up], HD speed and how full and fragmented the drive is, all effect performance.

Just for God's sake don't be tempted by these new Solid State HD's thinking that they will speed things up, I've installed about 20 in peoples computers over the last 5 months as a seperate drive with just the OS on them, and the failure rate is about 30% atm, and I expect it to rise, some have only lasted 2weeks.
Sure they are blindingly fast, about 25X -50X faster, and when they work it's like WOW! but as reliable as the first generation of Japanese cars.