What were the arguments against the dafyomi cycle proposed by Gedolim when Rabbi Meir Shapiro tried to found it?
According to this site,
the Lithuanian yeshivas [who] felt that the study of one page per day would become perfunctory and not convey the depth of Talmudic knowledge.
This site summarizes an argument but doesn't attribute it to any historical voices,
The breakneck pace means that the learning is often superficial and not committed to memory.
This youtube video has a discussion including historical opposition (starting at about 1:10) which gets to the ideas presented on the other pages I cited.
The following are some of the arguments of R. Hayyim Eleazar Shapira (Rebbe of Munkacz and author of Shu"t Minhat Elazar) a contemporary of R. Meir Shapiro who held that the Daf Yomi was ridiculous:
“For how can one learn a page every day when the pages almost always end in the middle of a subject.” Divrei Torah (Brooklyn, 1998), vol. 6, no. 82. Elsewhere he explained that the great danger in joining a Daf Yomi group is that one might be led to adopt the Agudat Israel ideology, "and Heaven forbid to join with them." Iggerot Shapirin (Brooklyn, 1983), p. 319. He also accused the Agudah of initiating the Daf Yomi in order to have at its disposal ready-made groups that could be used to colonize the Land of Israel. See Sha’ar Yisaschar (Brooklyn, 1992), p. 382. (Source see footnote 7).
Another rabbinic great who opposed it at the time, was the Satmar Rav, R. Yoel Teitelbaum. Like the Minhat Elazar, he suggested that it was part of an Agudist Zionist plot. As quoted in Kovetz Ginas Verodim (2011) of Satmar, (in turn quoted here), he said that that:
The Agudists use the study of Daf Yomi to show that they study Torah, but their intent was to cover up the bad that they do though this - strengthening the various heretical Zionists (Trans. my own).
This page also noted:
While I do not think that R. Joseph B. Soloveitchik can be called an opponent of Daf Yomi, I was present at a shiur in the summer of 1985 where he expressed his dismay that due to the growing popularity of Daf Yomi, people were no longer studying all six orders of the Mishnah, much of which has no Talmud and is thus not included in the Daf Yomi cycle.
At the time of the initiation of Daf Yomi, R. Soloveitchik was a rising star, but he probably wasn't a well known opponent of the time. The view of a near contemporary is nevertheless relevant.
Obviously, these aren't my views, and I am not endorsing them. I am answering the question of why some rabbis at the time, including the very influential Minhat Elazar and Satmar RAv, opposed it. This isn't the place to attack them, or their ideas.