The easiest way to guage this is watching other barrel roll on the girders above and on the girder in question. If they don't move apart from each other then it's constant. To me, they are constant, even though girders 3 and 4 may be steeper slopes, the 'x' value which controls the left & right movement stays constant. Now "ALL" barrels get a little faster at high level.
Of course if this was "real physics", everytime a barrel dropped to a new girder and changed direction, starting at speed '0', there actually be acceleration in the x direction affected by the sine of the angle and gravity. That would be cool see a barrel board with real physics applied.