The following are the relevant data I could find on the CSS Arkansas and the actions she was in.

Steam Pressure/engines: William N. Still Jr.’s “Iron Afloat” states the pressure dropped from 120 to 20 as the result of stack/breeching damage. Silverstone’s “Civil War Navies.” lists the Arkansas as having “2 low-pressure direct-acting engines.” High pressure boiler plus low pressure engines?

Steam Drum: In “Iron Afloat” the engineer is quoted as saying that on its final mission the steam drum was supporting the weight of the gun deck and that ramming would likely carry the steam drum away. I wonder if this was the result of damage suffered during the fighting getting to Vicksburg or while there. A “large bolt” had penetrated the engine room and caused a number of casualties. It might also have damaged support timbers. If there were structural problems with the steam drum mounting, one can't help but wonder if some additional stress was also being placed on the engine mountings. (My experience with a few large, old steam cylinder compressors as well as with turbines and pumps has been that alignment problems or piping in a bind tear things up rapidly.)

Speed: According to Luraghi’s “A History of the Confederate Navy” in engine trials the Arkansas did 8 knots in “slack water” and 4 knots against the current (on the Yazoo.) It took her an hour to run down the Carondolet--listed as having a speed of 5.5 knots (Silverstone.) I’m guessing that the Carondolet had little more than a one mile lead by the time she completed her turn so that the overtake was in the range of 1 knot.

The Tyler seems to have been able to stay ahead of the Arkansas even running in reverse. The Tyler was capable of 8 knots forward (Silverstone.) Taken altogether it seems most likely to me that the Arkansas could run about 7 knots tops if there was truly no current (despite the “slack water” trial that listed the speed as 8.) This still made her relatively fast for an ironclad at the time--certainly compared to the New Orleans built vessels that couldn't even stem the current reliably.