The KFR library is a high-performance C++ framework for digital signal processing (DSP) and audio applications. It provides optimized implementations of FFT/DFT, filter design (FIR/IIR), sample rate conversion, audio file I/O (supporting WAV, FLAC, MP3, AIFF, CAF, etc.), SIMD-accelerated mathematical operations, and tensor/multi-dimensional array operations. The library is designed for real-time processing and scientific computing, with architecture-specific optimizations for x86 SIMD, ARM NEON, and RISC-V RVV, and runtime dispatch for multiarchitecture support. Key architectural features include a header-only design (with optional separate compilation), expression templates for lazy evaluation and operator fusion, a portable SIMD abstraction layer, and a tensor system with NumPy interoperability. The library is used in production by scientific collaborations such as LIGO, Virgo, and KAGRA for gravitational-wave signal processing, benchmarked against FFTW. It employs a dual licensing model (GPLv2+/commercial) and requires C++20 for compilation. The codebase demonstrates strong engineering practices through reusable components such as the SIMD vector abstraction (vec<T, N>), expression template system, tensor class, and audio data handling unit. These components are self-contained, exhibit non-obvious skill in C++ template metaprogramming, and can be extracted for use in other projects with minimal modification. The library is actively maintained, with recent commits and CI/CD pipelines, and shows low technical drift by keeping pace with modern C++ standards and hardware advancements.