By Horav Yosef Yeshaya Braun, member of the Badatz of Crown Heights
There was a minhag prevalent in some shuls in pre-war Europe to have a hafsakah (break) after Shacharis on Shabbos, during which the congregants would go home to make Kiddush; they then would return to shul to read the Torah and daven Musaf.
However, there were people who did not wish to do so, since making Kiddush at this point in davening is halachically questionable. There are poskim who maintain that one may not eat before the Torah reading and haftarah. In addition, if one makes Kiddush before Musaf, they may eat only a kebeitzah (the volume of an egg) of mezonos.
As a solution to this problem—and because they did not have the possibility of arranging a separate Torah reading—some wished to daven Musaf first and then join the rest of the congregation for the Torah reading. Many poskim address the shailah of whether it׳s acceptable to daven Musaf before leining; there are those poskim who believe that it should be done only on rare occasions.
Making Kiddush after Shacharis is not recommended, and is usually done only on Simchas Torah. It׳s best to daven all the tefilos in their correct sequence and to make Kiddush afterwards, unless extremely hungry or thirsty.
(It׳s important to note that some of today׳s “Kiddush clubs״, in which some people leave the minyan in the middle of davening to make Kiddush and socialize, often lead to disorderly conduct and to disturbing the decorum of the shul. Additionally, quite often it takes place during the actual services, and the participants miss out on parts of davening, or they daven some parts without a minyan. Unfortunately, in many cases it leads to intoxication during davening—and it׳s explicitly prohibited to daven while intoxicated. In addition, there is the serious safety concern of substance abuse, as well as the negative impact this behavior has on children.)