The EU Stall % metric represents the percentage of time when the GPU execution units (EUs) were stalled. An EU becomes stalled when all of its threads are waiting for results from fixed function units (for example, a pixel shader requests texels from the texture sampler).
If EU Stall % is 50, it means that EUs were stalled for 50% of the rendering time for the selected ergs.
If EU Stall % is 0, it means that there were no stalls in EUs or the stall time is very small.
If this metric is unexpectedly high, especially when compared with the EU Active % metric, you can analyze where the stalls happen by looking at the VS EU Stall % | GS EU Stall % | PS EU Stall % metrics. If any of these metrics show that most of the stall time is in one particular shader, examine your shader code in the Graphics Frame Analyzer to determine why this shader might be causing the EUs to stall.