Let's start with the fact that the McArthur boasts a top speed--as advertised--of 12 knots. That's roughly 13 miles per hour. To put this in perspective, this is roughly equivalent to a WW2-era Liberty Ship, which had a maximum speed of 11.5 knots. The Maersk Alabama, a type of ship which might be escorted by the McArthur, has a top speed of 18 knots. Also, keep in mind that the McArthur is a paid alternative to ships of Combined Task Force 151, which include modern Arleigh Burke-class destroyers, with speeds of over 30 knots, and US Navy aircraft carriers can top 30 knots as well. Even the speedboat we had at our Boy Scout camp to teach water skiing merit badge could travel faster than 12 knots!
Indeed, not only is the McArthur not fast enough to even keep up with the ships it will be escorting, but it lacks the speed to respond to a distant emergency and intercept pirates who might be in smaller speedboats. Sailors who served on the McArthur even told stories about the ship's speed and suitability for the mission (scroll down). Face it, there was a reason this thing wasn't going to see service anytime soon.
*--in a previous post, I referred to the vessel as the MacArthur, as in Douglas MacArthur, but it appears that the vessel's namesake is a US Coast Guard officer named McArthur instead. The ship served as a hydrographic charting vessel in the US Coast and Godetic Survey.