FHSS is just the type of 2.4 technology Futaba uses. (Frequency Hopping Spread Spectrum) Futaba only have two systems, both are FHSS. (Required by certain countries such as Japan to use 2.4ghz.)
FASST is their trademarked system. All the air radios except 4YF uses it. 3PM/3PK/3VC/3GR/4pk are all on this system as well. The air and surface Rx have different GUID/protocols and are not compatible.
sFHSS is the budget system for the PL/4YF line. I believe the air/surface receivers are interchangable between air/surface radios on this system.
As for latency etc/ you'd have to get an official statement from Futaba. Or find someone with a lot of testing equipment