Have you noticed that major religions LOVE to organise synchronised days of worship, even down to the hour? The spirits who have set themselves up with full-on cults are basically being summoned (depending on the form of worship and personal beliefs of the devotte, that’s anywhere in the fields of internal invocation, and evocation to a specific location) and they like to have it done within a tight timeframe.
(They also like to claim a piece of territory for themselves, or a specific image, and then get that image replicated as widely as possible, and to dominate the visible world with buildings devoted to them. But that’s a slightly different topic, only mentioning it because it goes to the issue of attention and having people being made aware of them as much as possible, at once - so it indicates they don’t have any problem handling simultaneous calls on their attention.)
This system of setting up a specific day where their people summon them would appear to suggest that, whether they’re in some way a function of our minds (my old man’s belief - NOT mine) or whether they exist objectively seperate from us, and regardless of the possibility that they exist in a different relationship to space and time than we do (which I proposed as a possibility here), being called upon by several people at the same time is something they actively want.
Group workings and evocations I’ve been part of, dating back to my white light days, are always VERY heavy, very powerful, you can really feel the presence of the spirit.
If we’re all (my theory) thoughts held in the Mind of the All, and we all simultaneously hold a thought ourselves, we give it much greater heft and power to act within this world - I mean, to take a weird example, I have my Cabinet of Invisible Counselors and that’s been cool, but the fictional character Mark Watney, who’s held in the minds of tens of millions of people from the book and then movie The Martian, seems to have a lot more presence and agency than many of the others.
I believe, because he’s been held in so many minds, and for most people, with affection and even some degree of concern (for his fictional predicament) which is one heck of a mass of energy for any spirit to feast upon. (I’m working on the theogenesis of Watney, and will possibly come back to this in the near future.)
So, other spirits who are real (as in, their attributes were established before your birth and existed in the manifest world without your observation - and were not created in the mind of one man) probably get more of a boost, the more minds they have focusing upon them at once.
The fundie Xians are therefore probably technically correct that ANy fiction featuring demons (and also, non-Xian gods, who they just comnsider to be demons anyway) IS bringing them forth just a little more, each time.
Which I think is cool! 
Either way, I would never be concerned that a spirit is kind of like a distracted mum multi-tasking with less attention the more they;re called - I would say, based on the fact that they LOVE to be publicly thanked, names, have their seals inscribed in metal etc - and based on the curious case of Mr Watney - that the more people think about them, evenm in the same moment (in our perception of time) the more power they can bring.
A MASS evocation by a group of people, of a single specific spirit with a single intent, at the same moment (simultaneous, not adjusted for time zones) would probably be pretty mind-blowing, I think we’re a ways from organising something like that, but the potential intrigues me.