Accuracy is the crucial factor. Not speed. Speed is secondary.
Practicing counting down decks at max speed is useful because it requires concentration.
In a casino you will very often have to focus in order to block out multiple distractions.
If you cannot demonstrate near-perfect accuracy AND good speed, then you (yet) lack the requisite focus to justify investing cash.
Learn to find small chunks of spare time to reach into your pocket for that deck of cards that is your "constant companion".
I specifically suggest counting down cards face-up in groups of 2's, 3's and some 4's — grabbing these cards quickly and randomly.
Counting cards singly will leave you dependent on staring at the cards, dependent on the speed of the dealer's hands.
Leave two (2) cards uncounted — by removing them beforehand.
See if you can declare the possible hand(s) that the final two (2) cards represent.