The influence of key process parameters on the production quality of glass fibre-reinforced polymers manufactured by high-pressure resin transfer moulding (HP-RTM) was systematically investigated. Uniform ±30° braided preforms were infiltrated under controlled variations of vacuum level, resin discharge rate, and preform fixation. Non-destructive characterisation using air-coupled ultrasonic testing, computed tomography, microscopy but also visual inspection revealed process-induced defects such as in-plane fibre displacements, out-of-plane waviness, voids, and dry spots. Subsequent quasi-static compression testing showed that while Young’s modulus remained largely unaffected, compressive strength decreased by up to 26 % under suboptimal processing conditions and was accompanied by a transition from shear-dominated to diffuse, matrix-dominated failure behaviour. Optimal laminate quality was achieved by evacuating the mould cavity to below 1.5 mbar, applying a reduced resin mass flow rate of approximately 10 g/s, and stabilising the preform by dual-sided fixation, resulting in an in-plane fibre orientation of ±30° ± 1.7°. The results establish clear process–structure–property relationships and provide quantitative guidance for robust and reproducible HP-RTM processing.