When changing money in Namibia at banks, allow plenty of time. And, if you're actually cashing traveller's cheques, add some more time onto that. :)
There seem to be an insufferable number of forms to fill out, supervisors' approvals to obtain, and so forth. AND, if you're changing a "substantial sum" (which could be anything over about $300 US), it seems to triple the time and paperwork.
You can avoid the delay in two ways. First, changing money at a hotel can be quicker.....but you'll pay dearly on the exchange rate. I don't recommend this plan.
THE best plan? Use your credit cards as much as is possible. For the most part, this is a seamless way to pay for something if you don't wish to deplete your Namibian cash. And, you get pretty close to the official exchange rate, or so it seemed to me when our bills came.