Comment by rastapasta

Even though it runs locally, it is network-connected to the tile server that provides you with the vector tiles (unless they are cached locally after their first use). You currently have to wait for the requested tile(s) to be downloaded, processed, and available in memory before continuing rendering (which is slow in direct comparison to rendering itself). Could be improved by continuing to render the zoomed features of the parent tile until its children are ready (triggering a re-render) - but for now, it simply lets you wait until then :)