We report the results of a systematic study of the time performance of triple-GEM detectors operated with Ar/CO 2/CF 4 and Ar/CF 4/iso-C 4H 10 gas mixtures. This study and the detector development has been performed in the framework of an R&D activity on detectors for the innermost part (region R1) of the first muon station (M1) of the LHCb experiment. The use of fast gas mixtures allows to achieve a time resolution better than 5 ns (r.m.s.) and a single detector efficiency up to ∼ 99% in a 20 ns time window. Discharge studies have been performed at the πM1 hadron beam facility of the Paul Scherrer Institute. Local aging properties were investigated exposing the detector to a collimated 5.9 keV X-rays beam. The whole R&D activity with different gas mixtures has been performed with small 10×10 cm2 triple-GEM detector prototypes. A full size prototype (module-0) constituted by two 20×24 cm2 triple-GEM detectors, equipped with front-end electronics based on ASDQ chip, has been built and successfully tested, with the chosen Ar/CO 2/CF 4=45/15/40 gas mixture, at the electron test beam facility at the Laboratori Nazionali di Frascati.