The term ‘fast FLIM’ is often misleading since it is used for different aspects of a time-resolved TCSPC measurement. The most important parameter in a TCSPC experiment is the time resolution on the picosecond to nanosecond time scale. In the context of TCSPC ‘fast’ suggests a high time resolution. That is the capability of the device to differentiate between fast fluorescence decays, i.e. short fluorescence lifetimes. Often, however, the term “fast” is not used to refer to high precision, but rather to fast data acquisition or a short measurement time.
According to the above, ‘fast FLIM’ means a FLIM experiment with a short acquisition time. The purpose of such a system is often to be able to record FLIM at video rate. Of course, for this you need to use higher laser powers and a photostable sample (usually achieved by staining, not suitable for autofluorescence). Often, such systems (physically) sacrifice the time resolution on the picosecond time scale and by that achieve the opposite of fast or high precision TCSPC.
Becker & Hickl has developed a ‘fast FLIM’ system that allows both high count rates for video-rate FLIM and also has an enormously high temporal precision. These time taggers can be found here.