Not sure why it would take so long and don't have a Roku tv to test... but why are you calling bitmap.Finish() if you have not drawn anything to the bitmap? I do not use bitmap.finish() and have never had an issue. If you are using a roScreen with double buffer I think it will do a finish before swapbuffers(). You will only have this "execution time" hit when creating. Draws are done quickly.
2nd possibility is that the new Roku tvs' and roku ultras do have new chips and draw2d drivers have not been implemented exactly the same as the older devices. I have a new Roku ultra and have not encountered any issues with slow bitmap creation that I have noticed but there is a scaling issue with non HD roScreen.