Sounds like there is a draw issue, or the thermometer you are using isn't accurate.
Possibly too little intake, too little exhaust, or a combination of both. Or just a bad thermo. I can get my UDS up to 300 in 15 minutes after adding the lit coals, and do nothing special to get there.
Pics of the UDS might help us figure it out.
Modded Brinkmann Gourmet. Basic UDS. Rescued 22.5" OTS. SJS Mini-WSM. Stubbs. B&B Oak Lump.
Cannondale 2009 F8 (for burning off all the great BBQ)