"The machine must be kept operating at its *WORD* " - What is the word?

The machine must be kept operating at its ______.

I am trying to find the ideal word to complete this sentence.

What I am hoping to communicate is that 'the machine' must be kept in "tip-top" (or "perfect") condition in order to ensure it operates at maximum efficiency. I have tried looking for synonyms on words such as "prime", "apex", and "pinnacle", but this was to no avail.

I am almost certain that there is a professional-sounding word that could complete the sentence and communicate the above information in a clean, crisp, & concise manner.

Solution 1:

The first word that occurred to me was optimum. Four major dictionaries suggest that this is a good choice:

Merriam-Webster definition 1 is “the amount or degree of something that is most favorable to some end”.

Cambridge defines it as “best; most likely to bring success or advantage”.

Collins says “The optimum or optimal level or state of something is the best level or state that it could achieve.”

Oxford defines it as “Most conducive to a favorable outcome; best.”

Solution 2:

One common word often used in such contexts that you seem to have overlooked is peak:

constituting the highest or maximum level, volume, etc.; optimal; prime:

a machine running at peak performance.

[Dictionary.com]

This word finds frequent usage in the physical world, and so might serve OP's purpose well:

{Physics} the maximum value of a quantity during a specified time interval:

a voltage peak.

[Dictionary.com]

Another good choice would be the phrase in full swing.

If something is in full swing, it is operating fully and is no longer in its early stages.

When we returned, the party was in full swing and the dance floor was crowded.

[Collins]

Also this

firing on all cylinders

To be functioning or operating at the most desirable or greatest possible level of efficiency, speed, or productivity.

The automated messaging process is firing on all cylinders now that we've solved the bouncing issue.

I'll be firing on all cylinders after I've had my coffee.

[The Free Dictionary]