Should a big research (e.g., PhD) be published as one comprehensive high-impact paper or several smaller, faster publications?

In my opinion, both small and consolidated significant insights are needed in research. A comprehensive dataset will provide visibility and recognition, whereas publishing several small papers increases your citation and track record, which are essential for a scientific career. 

It depends on the amount of data generated, as well as on the depth of the data. Some studies simply do not have enough data to stand alone much less to be published as a paper. On the other hand, some studies simply have too much data and publishing it as separate papers would be more feasible. In summary, it depends but personally I would publish a single dissertation into two papers max. 

Usually I don't recommend salami slicing. A PhD is likely to have single chapters deserving separate publication. For example a health related PhD may have a literature review, clinical trial, plus a questionnare survey. All of which contribute the whole, but of interest in their own right. 

The question of whether to publish a large research project, such as a PhD, as one comprehensive, high-impact paper or as several smaller, faster publications is a common dilemma for researchers.

Publishing a single, comprehensive paper can create a significant impact, showcasing the full scope and depth of the research. This approach is often impressive for thesis defenses and job applications, and it can attract considerable attention if accepted by a top-tier journal. However, it typically requires more time for preparation, review, and publication. There is also a risk that if the paper is rejected, a substantial amount of work remains tied up in one place.

Alternatively, publishing several smaller, more rapid papers allows for the incremental sharing of findings, the building of a publication record, and the gathering of feedback from the research community along the way. This can help establish expertise in a particular field and make the work more accessible. However, individual smaller papers may be perceived as less impactful, and there is a potential risk of 'salami slicing,' where the research is divided too thinly.

In practice, many PhD projects benefit from a mixed approach. Researchers might begin by publishing smaller, focused papers to disseminate their work and then synthesize their findings into a comprehensive review or capstone paper. This strategy builds momentum, provides early career benefits, and still allows for a publication that presents the overall picture.

Ultimately, the most effective strategy depends on the specific field, the nature of the research, and the researcher's career objectives. 

if the university and supervisor provide sufficient guidance and resources then Q1 journal publication is a most. However, this also depends on the student's major and field of study, as some fields are extremely difficult, while others make it easier to achieve Q1 publications. achieving Q1 publications is an approve that the student are good enough to get a PhD degree. So that I agree that Q1 should be a standard.

Depends on goals and field; a single high-impact paper can be a prestige maximizer, especially for a PhD, but having several smaller articles increases your chances of having a high number of publications and prevents you from being seen at all. If the research is very novel, it is better to publish in one high-impact paper; however, if there is less time to get the achievement seen, better to aim for multiple small papers.

In my opinion it depends on the cons and prons of the research paper. Depends on:
The type of journals it published
The long term influence of the paper in the area
The risk of rejection and 
Times take to publish

In my opinion, quality is more important than quantity. So, one precise, fluent, and comprehensive paper is more valuable. 

The PhD thesis should preferably be divided into more than one paper, especially since some chapters may differ from each other in their scientific content. 

The current model for scientific publishing can accomodate either situation, however it is more common to find journals accepting focussed publications than longer, integrative ones. The impact factor of many journals of the latter type can be rather high. The rest is strategy. Scientific progress can come from either situation, you are supposed to read everything in the literature; however it is easier if you find lots of info on a single source. Public understanding can gain from either, however in my opinion well placed and interpreted snippets of info can do better than a single monolithic appraisal of a source (the general public tires so easily). In terms of accesssibility, it is all the same with so much variation among journals from free access to high cost paywalls. For a student, again it can be either: a Nature paper is surely the star of a CV, however many papers mean more experience with publishing and making sense of specific scientific findings. Make your choice.

PhD should publish a couple of high impact papers. No need to rush and publish several, and it is a PhD again no need for a big paper that will take many many years

Often it is not possible to publish one big paper for teh PhD reserach as it will eventually will become too big and cannot be accomodated in one paper.  Thats why the work is normally divided into around 3 to 5 sub papers, in this way a comprehensive detailed discussion is possible.