The shield wall "speed" was just an idea but it is fact that the phalangites movement was much slower.
In the wargaming community we have the rules which tells us that the phalangites movement is usually 3-4 points(inches) per turn and in the same time a much quicker hypaspistai have 4-5 points and the cavalry 8-9 points(depending on the quallity of the unit and its armour),
also in the rules we state that the phalangites can not make 90° face turns, they can only push forward (there was not one battle in the history that the phalangites have used 90° face turns to left or right) but of course this is not a wargaming forum and the RTW engine is very limited.
Aemilius Paulus i can send you some wargaming documentation about this if you are interested in and for others who do not know what the wargaming is, here is one good example:
http://www.fieldofglory.com/features/1.html
Battle of Magnesia
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