Supershots in TFTR often suffer a performance deterioration characterized by a gradual decrease of the DD fusion neutron yield and plasma stored energy after several hundred milliseconds of auxiliary heating. The correlation between this performance deterioration and the development of low m (the poloidal mode number), n (the toroidal mode number) MHD modes is studied through shot-to-shot comparisons and statistical data analyses. A good correlation is observed between performance deterioration and the appearance of strong 3/2 and 4/3 macroscopic modes. The magnetic island structures are observed using Mirnov and ECE diagnostics. The measured Te, Ti and ne profiles show that development of the islands corresponds to a nearly constant decrement of these quantities over the core region r < rs, where rs is the mode rational surface, on a transport time-scale (t > τE). The observed energy deterioration scaling, δW/W varies as w/a, where w is the magnetic island width and a is the plasma minor radius, agrees with a local transport model. Numerical simulations based on the local transport model reveal many features consistent with the experiments. Besides the MHD effect, it is found that a continuous increase of edge recycling rate during the neutral beam injection phase also has a large effect on the performance deterioration