One thing i do sometimes is when a card comes out i wont count it UNTILL i see the next card that has a point value, if the card negates it, i just skip it and move onto the next card to come out, its liek counting in pairs which i do sometimes, but this is for when you are counting the cards as they are dealt not after they are already in pairs on the table.
In terms of how fast you can count, i personally have trouble physically dealing out a deck in less time than 27 seconds, so idk if some of the crazy numbers people say you must be able to get to, like 15-20 seconds are neccessary 100%, if your slow, just play at third base as it gives you extra time to count or recount if you have to before its your turn.
In general you should be able to count in terms of the final product only example: +12,+13,+14+13+14 as the cards are dealt, it important to practic counting in the double digit range as you will sometimes find rc in the 30s in 8 deck games while as your practice with a single deck may only yeild numbers as high or low as 12ish so if your practicing with a single deck, try starting the count at 22, and returning it to 22 or some other higher or lower number than 0, just to get used to ading and subtracting numbers in the teens and twenties efficiently.