There is no one way to do it. With full focus (which is how everything spiritual should be done), it's like this in descending order of power:
:arrow: Long, drawn out vibrations to your full capacity without straining yourself
:arrow: Medium length vibrations
:arrow: Short vibrations
:arrow: Whispers
:arrow: Mental vibrations
Some may not be necessary but unnecessary doesn't mean useless.
High numbers are also not all that matters. 8 fully-drawn out vibrations will always be more powerful than 80 short vibrations and 666 mental vibrations or whispers.
Each number has their specific function. Low numbers are not to be looked down upon. 9, for example, has a very different function compared to its multiples 18, 27, 36, 45, 54, 63, 72, 81, 90, 99,108...111... 144... 216... 333... 666. All different functions. They are not interchangeable, as that's not how mathematics works.
If you get tired easily, you're definitely overdoing it. Remedial action can be either to reduce the number of chants and keep the speed, or keep the number and change the speed. Both are effective. Which course of action is the most effective will depend on your undertaking. What is certain is that, at lower levels, the difference won't matter much.
I see too much emphasis is given on quantity nowadays. There certainly is power in numbers, but there is also power in quality. So you should find the balance of probabilities that is most appropriate to your situation. Personal circumstances are also important, not just the type of working/meditation you are doing.
If you don't understand how quality can affect something, allow me to share an example. If a particular fighter is as strong as 10 fighters, who would you pick to fight? It obviously depends on the situation. In one situation, it may be better to use that fighter. In another situation, you may pick 10 fighters who are worth as 1 as they are more appropriate to the circumstances you are facing. If you need to build the strongest team, you would however build it with fighters that are worth more than 1 fighter.
In Vedic India, warriors had different classes depending on their worth:
:arrow: Rathi, a warrior capable of taking 5000 warriors simultaneously
:arrow: Atirathi, a warrior capable of taking 12 rathi class warriors or 60k warriors simultaneously, and master of all forms of weapons and combat skills
:arrow: Maharathi, a warrior capable of taking 12 atirathi class warriors, 144 rathi class warriors or 720k warriors simultaneously (i.e. Karna, Ravana, Naraka, etc)
:arrow: Atimaharathi, a warrior capable of taking 12 maharathi class warriors or 8.64M warriors simultaneously (i.e. Indrajit, Hanuman, Rama, Krisna, Arjuna, several forms of Siva, and avatars of Visnu)
:arrow: Mahamaharathi, a warrior capable of taking 12 atimaharathi class warriors or 207.36M warriors simultaneously. (i.e. Lord Siva, Visnu, Brahma, Sakti, Ayyappan, Murugan, Vinayagar, Panchmukhi Hanuman)
A more modern example can be comparing various types of weapons. For example, artillery to explosives to rifles, etc. Or how much power has a fRTR done by Lydia, compared to an RTR done by a newbie. Same reps, extremely different quality and output.
There is no clear-cut answer to your question. What's clear is that you were overdoing it.