Wadsley-Roth oxide FeNb11O29 powder samples are successfully prepared using a simple, cost-effective, and ultrafast microwave-assisted solid-state synthesis for the first time. While conventional solid-state route in furnace requires hours of high-temperature treatment, both monoclinic and orthorhombic polymorphs of FeNb11O29 were obtained in minutes under microwave heating. Combining such short heat treatment with submicrometric oxide precursors enables to limit particle growth during the synthesis. The electrochemical benchmark clearly shows that FeNb11O29 powder samples obtained rapidly from submicrometric oxide precursors exhibit enhanced cycling performance. For example, the monoclinic polymorph prepared in only 5 min offers a high capacity of 179 mAh g-1 (90 % retention) after 500 cycles at 2 A g-1, approximately 20 % more than with conventional synthesis protocol. Electrochemical analysis demonstrates that the extra capacity is gained at low voltage and is probably induced by an easier ionic diffusion occurring in smaller particles. This work confirms the interest of solid-state microwave heating to design of electrode materials with limited particle growth and better cycling performance.