Since the upgrade to Webpack 4, overall build time has increased. This may be in part to due to the mode configuration of Webpack 4 which includes several optimizations enabled by default. There may be some settings worth tuning.
Along with the Webpack 4 upgrade, a new favicons-webpack-plugin was introduced to retire some logic for branding that had been implemented in a way that was against plugin recommendations for Webpack 4. This new plugin provides the additional feature of generating icons for multiple device types. As observed by cherewaty, this process may also be reducing build performance.
We can provide separate settings for dev builds so as not to process icons, or perhaps eliminate the branding plugin from dev builds entirely.