We utilize the bulk viscosity of interacting strange quark matter to reevaluate the damping timescale. The presence of a medium effect of bulk viscosity leads to a stronger damping of r-modes, which can be over an order of magnitude for realistic parameters. We find that the r-mode instability window is narrowed because of the medium effect, and hence, when a pulsar reaches the instability window, it will slow down by gravitational wave emission to a period of only 1.78ms instead of the 2.5ms given by early estimates. As a theoretical upper rotation limit of pulsars, the period of 1.78ms is very close to the two most rapidly spinning pulsars known, with periods of about 1.6ms.