I just installed Comcast high speed internet which is not $19.99/mo one on Comcast's website but $29.99/mo for first 6 months, and my speed is 12Mbps not 15Mbps. Why I pay more but get lower connection speed? Since mine is cable connection, can I watch TV?
The 19.95 offer is for existing TV and / or Comcast phone subscribers to add Comcast HSI internet to their service if they meet the terms and fine print. That price is good for 6 months.
You don't really say what you signed up for and if you had TV or not. If you signed up for new internet only service, then you should not get any TV service, TV should be trapped on the drop line. You could also have signed up for internet & basic TV, which would get you the most basic B1 tier of TV channels without a set top box.
There are basically 4 speed tiers. Add 4mb for powerboost.
You probably have performance and normal download speeds would be 12mb.
And let us know what HSI tier your signed up for, you might need some troubleshooting help which the experts here can help with. If you have a router, try testing it without the router and see if it improves. Removing the router, you will need to reboot your cable modem for it to detect the new network device, as it stores the old mac address in it's settings, and that needs to be cleared out changing devices.