I've had good and bad experiences with both URC and RTI.
Once the DTV box acknowledges the channel, execution time is up to the DTV box, not the remote.
I don't do much with DTV. I have observed some of the local cable system boxes have odd issues -- causing them to slow down or crash. A cable box reboot (remove power) can sometimes cure an issue -- for a while. Replacing the box is eventually required, but this might trade a headache for an upset stomach.
Are you keying in the channels digit by digit or by macros (A key for "NBC" or similar). If you are using macros, it is possible that there is something odd about the command timing that is frustrating the DTV box -- requiring a few seconds to recover. The MX850 and the outside remote are separately programmed and the command timing might accidentally be different on the two remotes.
This is a wild suggestion, but some phone and tablet cameras can see IR. If you have one of these, watch the IR transmission to the DTV box and try to see if there is any difference between MX850 and outside remote transmissions. This may not be so easy. If you are using macros, there may be a stray command at the end of a channel change or poor command timing that is upsetting the DTV box.
I would reboot everything before getting too excited. Also, I've seen some odd events associated with remote batteries that are near or beyond the end of life.
