The Mishnah in Avos (6.6) tells of difference ways to acquire Torah. Here is Rashi’s commentary on one of the items, translated by Sefaria.org.
בַּעֲרִיכַת שְׂפָתַיִם. שֶׁאֵינוֹ מְגַמְגֵּם בְּדִבְרֵי תּוֹרָה אֶלָּא חוֹתְכָן בַּלָּשׁוֹן וּמוֹצִיאָן בַּפֶּה, דְּאֵין דִּבְרֵי תּוֹרָה מִתְקַיְּמִין אֶלָּא בְּהוֹצָאַת הַפֶּה, דִּכְתִיב כִּי חַיִּים הֵם לְמוֹצְאֵיהֶם אַל תִּקְרֵי לְמוֹצְאֵיהֶם אֶלָּא לְמוֹצִיאֵיהֶם בַּפֶּה:
Preparation of speech: that he not stutter in words of Torah, but rather enunciate them with the tongue and bring them out with the mouth. Since words of Torah are only preserved by their being put out with the mouth, as is written, (Proverbs 4:22) "They are life for those that find them" - do not read for those that find them (motsaeihem), but rather those that bring them out (motsieihem) with the mouth.
I wonder, then, how this compares to the subject of subvocalization. Wikipedia and others websites discuss the value in its removal, in order to be able to read faster.
Advocates of speed reading generally claim that subvocalization places extra burden on the cognitive resources, thus slowing the reading down. Speedreading courses often prescribe lengthy practices to eliminate subvocalizing when reading. Normal reading instructors often simply apply remedial teaching to a reader who subvocalizes to the degree that they make visible movements on the lips, jaw, or throat.
I would imagine that there is a math problem one could devise on how many WPM Jewish scholars of recent times would have needed to be reading at in order acquire the mental libraries they did. Such would be people like Rabbi Yosef or Rabbi Kaplan. (Or not so modern times, Rabbi Eiger or Rabbi Karo) But there are countless others certainly.
Are we to suppose that this is an important rule, or must it not be followed under the right circumstances, or only sometimes? Otherwise, I don’t see it plausible that this rule was always in effect. For how could one read that much text, that quickly, with subvocalization?