If you have flashed a custom ROM on your HTC One S and subsequently noticed a drop in your HSPA+ download speed, you’re not the only one. Several users have reported speed drops to 3-4 Mbps from 10-14 Mbps after flashing a custom ROM. Luckily, we now have a solution that should bring your data speed back to the HSPA+ levels.
When XDA Forum Member TechieGeek started experiencing low download speed on his One S after switching from stock to CM10, he found that the problem was due to baseband compatibility issues. Flashing a new baseband and changing some settings in the build.prop fixed the issue. Judging from the response, the method seems to work for those who try. However, this will only help if your carrier is actually capable of delivering speeds greater than the lower values.
The complete instructions can be found in the forum thread, so head over and get your HTC One S back in the fast lane.