Fast, sensitive and selective gas detection is vital for applications ranging from environmental monitoring and industrial process control to medical diagnostics. Optical detection techniques, particularly those operating in the mid-infrared, offer high selectivity and sensitivity but often suffer from complex alignment. This work presents the design, fabrication, and experimental validation of a novel antiresonant hollow-core fiber (ARHCF) with a large core diameter (107 µm) optimized for rapid mid-infrared gas sensing. The fiber features a simple geometry with 12 thin-walled silica capillaries forming the cladding, ensuring effective single-mode propagation at 3.3 µm and high flexibility. Experimental characterization confirms broad transmission bands up to 3.4 µm. When applied to methane detection using a 3.3 µm interband cascade laser, the system achieved a detection limit of 4 parts per billion with 1 s averaging time. Crucially, the enlarged core drastically reduced gas filling and response time to as low as 2.2 s for a 5.2 m fiber. The proposed large-core ARHCF effectively overcomes the trade off between sensitivity and response speed in hollow-core fiber-based gas sensors.