Photon upconversion (UC) based on triplet-triplet annihilation (TTA) is an emerging wavelength shifting technology, which is applicable to sunlight. Previously we found that the quantum efficiency of TTA-UC (Phi(UC)) carried out in ionic liquids (ILs) is dependent on the type of IL employed. In this article we investigate the kinetics of the triplet emitter molecules (perylene) that implement TTA to determine the origin of the IL dependence of Phi(UC). We measure the time-resolved delayed UC fluorescence intensities from samples made with five imidazolium-based ILs, and their intensity decay curves are analyzed with an analytical model. Consequently, several important aspects regarding both the first-order and second-order decays are elucidated. It is revealed that the IL dependence of Phi(UC) primarily originates from the IL dependence of the branching ratio toward TTA upon an encounter of two triplet emitter molecules. Additionally, a strong correlation between the viscosity of the LLs and the branching ratios toward TTA is found. This finding is supported by temperature-dependent measurements, from which Phi(UC) is found to be significantly affected by the viscosity of the IL. The results of this study should provide a clue for further improving Phi(UC).