Low data speed for Xperia Z C6602 HSPA+

I just switched from the $30/month 1500 talk + text + 30MB data to the $30/month UL text + data (first 5GB at 4G) + 100 talk and I immediately noticed a significant drop in data speed even though my area (63121) is pretty good with T-Mobile coverage. I used to have a Download/Upload speed of 3Mbps/1Mbps (according to speedtest.net) with the old plan and now it barely gets up to 80Kbps/100Kbps. Isn't the speed supposed to be better with my new plan?

I'm currently using the Sony Xperia Z C6602 HSPA+ and here's the phone's specs taken from gsmarena.com:

HSDPA 850 / 900 / 1700 / 1900 / 2100 - C6602