彼は仕事が遅い(効率が悪い)。って英語でなんて言うの？

When talking about how someone does at their job then you can say 'he's not very good' inefficient means they are not working as well as they could, if they are slow at working then you can call them a 'slow worker'

When you want to say that someone isn't very good at their job, You may say they are "inefficient". This basically means that the person doesn't try to their best ability. However, usually saying this sort of thing behind someones back is impolite, they have a personal reason they are inefficient.

To express that someone does not work in a satisfactory way or in a way that brings progress we can use these phrases. When someone is slow or unproductive they don't bring progress into the work they are performing and therefore they are not an asset to the company or the work they do.

"He is Inefficient." is more of a formal and politer way of referring to someone who is not sufficient/good at their job" "He Isn't very good at his job." Is a more casual and relaxed way of explaining this. "He is not a 'grafter'" is a slang term used when describing a lazy worker, 'graft' is referred to as doing work (In slang), Grafting,Graft,Grafter even 'grind/grinding' is a very similar and well used slang term.

Some workers are basically unable to achieve maximum productivity either because of lack of enough knowledge of the tasks they are allocated to perform or because of sheer laziness. Such workers can be described as being 'inefficient' or 'incompetent' and can't be relied upon. It is usually difficult for employers to retain 'inefficient' workers because they reduce the company's productivity levels. So, if you have such a worker and it's time to give salary increases or promotions, you may say: He/she is such an inefficient worker that he doesn't deserve to be promoted. or I can't recommend him/her for a salary increase because he/she is a lazy and unreliable worker. or He/she doesn't deserve a promotion because he/she is extremely incompetent and can't be relied upon.