his is a common issue that i have experienced, i am the enterprise manager for my company's Communications Department, this issue has to do with the patches applied by the system update, to correct this, provided your rooted and on the stock 2.3.4...
1. Get
MSL Reader from the market to obtain your
MSL Code its a 6 digit code, write it down
2. Reboot your Device go into a custom recover like TWRP and choose "Wipe cache" then "Wipe Dalvik" do this 2 times then reboot your phone.
3. On your dialer enter *#*#
DATA#*#* or *#*#3282#*#* click edit enter in that 6 digit
MSL code tap on Advanced, menu ad choose "Restore Defaults" the "Commit Changes" wait for Device to reboot.
4. Profile Update
5.
PRL Update (Optional)
The run your speed tests, when we did these steps our data speeds returned to Normal, if not faster, Hope this helps.