Good solution (usual solution, wastes 1 scanline of rastertime). But this is why I always say to tick "Cycle Exact" box
else WinUAE runs as fast as possible and now with superfast PCs a whole demo can run in under 1 scanline of rastertime. If you set Cycle Exact, you can code your effects and know (and see) how fast they will run on a real A500.