You know, i have the same problem
I can't seem turn of the cards that fast either. If I start coming close, the stack of cards will spill over and create a mess. Naturally, I take the time to straighten the pack, and but then no hope of finishing the count in 25 sec or less.
Two places I have seen the 25 sec benchmark. One is in Blackjack Attack by Don Schlessinger, chapter 7 I believe, where the performance standards for being accepted to a blackjack team are presented in a sample team handbook. The other is an article by Porkbeli, dated 98/02/03, that can be found at the novice corner at www.bjmath.com/main. He too talks of a 25 sec goal for counting down a single deck.
Here is what I have been able to do. I use the Hi-Lo count. Instead of flipping the cards, I pass them from my left to right hand in a fan style and count them as they pass before me. And for a single deck I hit 18-20 secs consistently, and meet the 25 goal always. (That is with my removing 2 cards face down to check on my accuracy.) Using the same technique, I usually hit 40-45 secs without error with two decks, and always make 50 secs (here I pull 4 cards to check myself). It took me a few hours of practice, spread over a number of weeks to get this fast.
Currenly, I have no problem keeping up with the count during play at the tables.
And I also practice with Casino Verite with the speed setting up high. Single and double decks are no problem, and shoe games on CV I am off by one point about 1/4 of the time after 3 decks in a 6 deck game with 5 players. For now, I stick to pitch games at the table, although the speed of actual games is far slower that the CV practice so I feel confident of success at shoe games.
One other thing I do to practice counting is watch Celebrity Blackjack on the game show network, counting as the game goes on. If you've tried it, you'll know that each cards is only on camera for a second or two...I think it makes for good practice. Last night, I was able to keep up with the count despite my wife talking with me the whole time--and she wasn't talking about blackjack!
Anyway, I, as perhaps you do, have my doubts about the 25 sec measure as described in the sources noted above.
Good luck. And continue to practice, I do daily.