Capillary electrophoresis (CE) has emerged as a relevant technique for protein and biopharmaceutical analysis, as it combines high separation efficiency, sensitivity, and versatility. The use of capillary coatings, including successive multiple ionic-polymer layers (SMILs), reduces interactions between analytes and the capillary, further improving the CE performance. Nevertheless, separations done on SMIL coatings rarely surpass 500 × 103 plates/m. To obtain the best out of the CE, it is interesting to have a detailed look at the sources of peak dispersion. Separations of a mix of model proteins were performed on (poly(diallyldimethylammonium chloride)/poly(styrenesulfonate))2.5-coated capillaries at different electrical field strengths, leading to plate height H against migration velocity u plots that enabled a quantitative analysis of each contribution. Using this model, capillary lengths and injected volumes were systematically varied. For the first time, the contribution of sample electrophoretic heterogeneity to the total peak dispersion was deciphered for model proteins and a monoclonal antibody. Dispersion due to electromigration was seen to have an impact on plate heights in the case of triangular peaks of small molecules but not for proteins under the present conditions. UV and mass spectrometry detections were compared on the same capillary, providing valuable information on the impact of the detection type on separation efficiency. Close to 1 million plates/m were reached in the best conditions.