  The development of detectors with a high time resolution has been pivotal to
our comprehension of neutron stars and the accurate measurement of their
properties. While high-time resolution astronomy has become a standard in the
radio and the high-/very-high-energy bands, progress in the visible band has
been comparatively much slower. SiFAP2 is a high-speed optical photometer
mounted at the INAF Telescopio Nazionale Galileo. Its potential emerged with
the discovery of the first two optical millisecond pulsars: these are among the
most efficient particle accelerators and natural laboratories of fundamental
physics. Optical millisecond pulsations challenge the standard pulsar paradigm,
requiring innovative solutions. Higher photon counting statistics of optical
telescopes, compared to high-energy instruments, attain unprecedented
sensitivity for weak pulsed signals from bright accreting neutron stars, which
are the best candidates for still undetected continuous gravitational waves.
