The gamma space where colors are manipulated. Proper sRGB conversion to linear and back is very expensive.
Note: alpha itself is always kept as is and considered linear.
Colors are blended in storage space
without gamma-conversion beyond going float(fastest). It can look way worse than the other modes.
without accounting for sRGB linear part. Essentially nearly the quality of linear at much cheaper cost.
See Implementation
The gamma space where colors are manipulated. Proper sRGB conversion to linear and back is very expensive.
Note: alpha itself is always kept as is and considered linear.