Organic electrochemical transistors (OECTs) are widely used in biosensing and bioelectronics, due to their ability to convert ionic into electronic signals and their high transconductance. Stretchable OECTs are particularly suited for on-skin and on-organ bioelectronics, since they are able to record or transmit signals under mechanical strain. Most stretchable OECTs are based on the conducting polymer poly(3, 4-ethylenedioxythiophene) doped with polystyrene sulfonate (PEDOT:PSS), which needs to be appropriately processed to yield stretchable films. Here we report stretchable OECTs that are obtained by modifying the mechanical properties of PEDOT:PSS films via the addition of low-molecular weight polyethylene glycol (PEG), which acts as a plasticizer. The presence of PEG in the films prevents the formation of cracks under strain while maintaining a high electrical conductivity, thus resulting in improved electromechanical properties. In particular, the additon of PEG leads to a higher channel thickness and increased ion mobility in the films, thus resulting in stretchable OECTs with high transconductance and fast response time. This work shows that high stretchability, high transconductance and fast repsonse time can be simultaneously obtained in OECTs, paving the way for their applications in conformable devices at the human-machine interface.