A rapid annealing technique for soft magnetic Fe75.1Cu1Nb1.5Si15.5B6.9 and Fe83Cu1Nb4B12 tape wound cores is presented. Its effectiveness for creating a fine-grained nanocrystalline structure is demonstrated by measurements of the magnetic hysteresis loops as well as by structural investigations done by X-ray diffraction. The results show that the technique reduces the average crystallite sizes from about 27 nm to 17 nm for Fe75.1Cu1Nb1.5Si15.5B6.9, and from about 52 nm to 15 nm for Fe83Cu1Nb4B12. The coercivity after the rapid annealing is about three times lower for the Fe75.1Cu1Nb1.5Si15.5B6.9 alloy and up to one order of magnitude lower for the Fe83Cu1Nb4B12 alloy compared to the coercivity after the standardized long-time heat treatment. The improved heat treatment is outstanding in the way that it produces better soft magnetic properties in high-performance alloy cores within much shorter time than the standard core heat treatments.