The metric PS EU Stall % represents the percentage of overall GPU time that the EUs were stalled in Pixel Shader instructions.
This metric does not show total amount of stalled time in the pixel shader, but only the fraction of time when pixel shader stalls caused the entire EU to stall. The entire EU stalls when all of its threads are stalled.
If PS EU Stall % is 50% it means that half of the overall GPU time was spent stalled on Pixel Shader instructions.
If PS EU Stall % is 0% it means that no Pixel Shader was associated with selected rendering calls or Pixel Shader threads were not causing EUs stalls.
This metric is important if pixel shading seems to be the bottleneck for selected rendering calls. If PS EU Stall % accounts for most the EU active time, then to improve performance you may need to simplify the pixel shader.
If PS EU Stall % is larger than you expect and you are encountering slow rendering times, you need to concentrate on pixel shader code to find reasons for these stalls.