The eastward movement speed of Madden–Julian Oscillation (MJO) events simulated in a 30-year simulation on a global cloud resolving model, nonhydrostatic icosahedral atmospheric model (NICAM), following the atmospheric model intercomparison project (AMIP) protocol, but with a slab ocean, was analyzed and compared with the observation. The simulation reproduced the observed tendency of the MJO to decelerate when they are embedded within stronger Walker circulation, intensified by background sea surface temperature states with larger zonal gradients between the warmer western Pacific and the cooler Indian ocean and eastern Pacific. However, the simulated MJO events displayed a slow bias and occurred disproportionately during El Niño events. These biases were associated with an overestimation of the western Walker circulation cell strength, which was partially counteracted during El Niño events. Our results highlight the importance of accurately reproducing the mean atmospheric circulation for the realistic reproduction of MJO in long term simulations.