Se + verb is using the reflexive se, but this construction is almost always translated into the passive voice.
In Spanish, instead of having something acted upon by an unnamed subject, they just have it act upon itself. So while Se rentan los apartamentos may be literally translated as The apartments rent themselves, it is typically translated into the passive voice in english, The apartments are rented.
In this example, while you may translate it literally as It can do itself quickly, it would be translated as It can be done quickly.
You can read more about it here:
It's not passive in Spanish, it's just that the meaning of the verb is the opposite of what you expected.
The presence of the reflexive 'se' puts the verb in more of a 'middle' voice. For an example in English, compare 'I am called X' to 'I call myself X' - the former is in the passive voice while the later is in the middle voice.
While I understand that technically the Se verb construction is a reflexive mediopassive, I think that with linguistic pedagogy in mind (especially on a lower to intermediate level) it is more helpful to show what is meant by the contruction and how it is translated into English than to bring higher concepts and distinctions into play.
You can carry it out quickly would be translated as Lo puedes realizar rápidamente.
Se + verb is often translated into the passive voice in English; as,
- Se rentan los apartamentos - The apartments are rented
- Se sirve cena - Dinner is served
When you have the Se puede infinitive construction, the Se often puts the infinitive into the passive voice; as,
- Se puede realizar rápidamente - It can be done quickly
Typically when something is in the passive voice, unless the subject is explicit, you use it.
I think Se in this context if for when the subject is unimportant or unknown. I dont think it means "anyone". It is more a passive sentence construction, for example - Se habla Espanol = Spanish is spoken, who it is spoken by is unimportant or unknown, what is important is that it is spoken (here or they would not have the sign up). In English, we would interpret Se habla Espanol as We speak Spanish, but that is a figurative and active translation. Here is another good conversation about Se, in this case "Ese se mapa ve viejo." - https://www.duolingo.com/comment/193831$from_email=comment_id=2739203 - it is not reflexive (verse), it is the unknown or unimportant use of se with the verb ver. (or so I think.)
Poderse is an example of the reflexive passive use of the verb. Reflexive passive is used when no definite person doing the act is identified in the sentence. For example, if the sentence says "SHE is able to do it rapidly", it would be translated as "ella puede realizar rapidamente".
"It can be accomplished quickly" should be accepted.