I've got 24 Atlas locos with LokSound Version 4's & 5's (L's & XL's) with the motors wired in parallel. They run superbly at slow speeds (speed step 6 which translates to about 3 smph).
Some will buck or hesitate at speed steps < than 6 but putting a few cars behind them solves that. Others are smooth at speed step 2 or 3. This discrepancy is more a reflection of the cheap DC can motors than the decoder.