perf_stats: Change MAX_LAG_TIME_US to an appropriate value

25us is far too small, and would result in std::this_thread::sleep_for
being called with this as a maximum value. This means that a guest
application that produces frames instantly would only be limited to
40 kHz.

25ms is a more appropriate value, as it allows for a 60 Hz refresh
rate while providing enough slack in the negative region.