I don't think I've ever heard these terms 'slow rapid' and 'fast rapid' outside chess960. Just now I tried looking up "slow rapid" "chess" "fast rapid" and "slow rapid" "fast rapid" and then almost everything there is chess960.
In the 2019 WFRCC, the following I believe applies for the semi-finals and finals besides just the quarter-finals:
Each match of the quarterfinals consisted of two “slow” rapid games (45 minutes for 40 moves plus 15 minutes for the rest, without increment), two “fast” rapid games (15 minutes plus 2 seconds increment) and 2 blitz games (3 minutes plus 2 seconds increment). The games were weighted differently: 3 points for each slow rapid game, 2 points for each fast rapid game and 1 point for each blitz game.
The time control is so-called 'slow rapid': Each game is 30 moves in 25 minutes, plus 5 minutes for the rest of the game after move 30, plus 5 seconds increment per move starting from move 31.
Question: How does FIDE define 'slow rapid' and 'fast rapid' ?
Note 1: Actually about the rapid WCC, I read just now:
In order to promote viewership, time controls were set at 15 minutes per player, rather than the pre-2012 standard of 25 minutes.
- In chess, 25 is in the same category as 15
- But in 960, 25 is in the same category as 45.
Note 2: If rapid is just 10-59 minutes, then I don't think slow rapid just means above the midpoint of 35 minutes since '25 minutes per player for the first 30 moves, after which each player will receive additional 5 minutes on the clock and an increment of 5 seconds per move starting from move 31' converts to 32.5 minutes and is considered slow rapid.