High-intensity focused ultrasound (HIFU) therapy is a minimally invasive method for precise thermal tissue destruction. Most applications make use of spherical, sharply focused fixed-focus transducers. These allow only small ablation rates, thus the treatment of large tumours is a very time-consuming process. The present paper describes the design of ancillary lenses, which combined with a spherical focused ultrasound transducer can be employed to reduce treatment time. The ancillary lenses shift the phase of the focused ultrasound waves, so that instead of one ellipsoidal focus multiple foci are generated. Thus, a single sonication can achieve a larger ablated area. To estimate the therapeutic benefits of the designed lenses, the lesion development during sonication was numerically simulated. Two "optimal" lenses were made of polystyrene and acoustically analysed. Using the designed lenses the theoretical lesion rate was increased by a factor of 3.7-5. There was a good correlation between simulated and measured ultrasound pressure field. The lenses reduced the system efficiency by only 13%. Thus, ancillary lenses present a technically simple and cost-effective method to accelerate ablative ultrasound therapy.