The Belle II experiment at the SuperKEKB accelerator is the new-generation intensity frontier electron-positron colliding beam experiment. The electromagnetic calorimeter consisting of 8736 CsI(Tl) crystals with their PIN-photodiode readout, has been inherited from the Belle experiment. In order to achieve high rate capability and immunity against beam background, the electronics has been upgraded to feature waveform sampling readout to provide not only energy deposition but also timing for each crystal hit. Design and construction of the upgraded electronics are reviewed and the performance in the early Belle II physics runs is reported. We also present the first application of calorimeter pulse shape discrimination at a B-Factory experiment. This novel technique, enabled by the new waveform sampling electronics, allows for discrimination between electromagnetic and hadronic showers, leading to significant improvements in kaon-long vs. photon separation and charged particle identification.