It's up to the employer to decide for how long they contract someone, whether they are more or less efficient is something that is their problem, not the employees' problem, they are reserved for more time so they should get paid more. Lots of jobs are paid by the hour, and that is the sort of job that I believe we were discussing.
The variables then are: your hourly rate, how many hours you are contracted for. I've never seen 'efficiency' in a labor contract as a metric, though, performance reviews may indicate that compared to your peergroup you are not performing well. In the end that then becomes an economics exercise, which in turn might lead to the company deciding that employing people for 32 hours is more efficient.
But until then they should pay for what they receive in terms of time spent on the job.
Your question presumes that my opinion on this matters but that's not how employment works. Employment is governed by contract law/labor law. To reduce your example to absurdity: Person 'A' gets their work done in 40 hours a week and person B in one minute, do you think person 'A' should get more money?
Clearly, the answer is yes because the claim on the life of person 'A' is a much higher one than the claim on the life of person 'B'. And you could argue the opposite just as easily: Person 'B' should get more money because they've figured out a way to be much more productive that might scale to others, and besides, sometimes getting something done in one minute has immense value and getting the same thing done in a week has none. But in practice neither of these things will happen. There isn't really a free market when it comes to jobs, employers hold most of the power and the differences between employees for most jobs do not exceed the normal bandwidth available for compensation, almost nobody except a lucky few contractors get paid by the value they create for the company.
So what will happen instead is that employers will set a certain hourly rate for a certain kind of job with some leeway to incentivize people to come and work for them or stay and hopefully a career path so they can migrate into better positions if they perform well. This arrangement will be formalized using an employment contract which together with applicable law will govern the relationship. This is not without problems but in the end it is what we have as a solution. Whether your colleague (who gets their job done in 32 hours) and you (who work 40 hours) have different pay depends on who is in control of the relationship: if the employees have control then they might be able to get paid by the value they contribute but this is extremely rare for salaried employees. The alternative, an hourly wage is much more common.
And one of the fundamental reasons for that is that we (1) all recognize the value of our time and (2) have tried very hard to get away from labor that was paid 'by the piece', which was very common in the early industrial revolution days (and which is now returning through a backdoor as the 'gig economy').
Someone who’s 25% less efficient at their job should get paid the same as someone else who’s more efficient?