According to Prof Moshe Gavra (מחקרים בסידורי תימן vol 3, p 297), although the practice to read Ruth isn't found in early Yemenite maḥzorim, the minhag has spread since the 18th century. Rabbi Yaḥya Ṣalaḥ (מהרי״ץ) wrote that they had the custom to read Ruth before minḥa. The common practice among Baladi Jews and many Shami communities is to read Ruth at home and then again at synagogue before minḥa (as they do with Ḳohelet and Shir HaShirim).
An article by Chaim Simons in Sinai 118, pp 26–42 (specifically pp 29–30) describes some other practices:
- Many diaspora Sepharadi communities (eg de Sola Pool maḥzor, Sephardic Heritage Foundation's Azharot book) have the practice of splitting Ruth into two and reading them after Azharot (between shaḥarit and musaf) on the two days. See also Soferim 14:18 and the old Romaniote rite (מחזור רומינה).
- The Algerian practice is not to split it into two, but read it on day 2 after minḥa.
- The Ashkenazi practice is to read it on day 2 before Torah reading. See Peri Megadim Eshel Avraham OC 490:8 for some (speculative) reasons: (a) the Torah was given on 7 Sivan, (b) people are tired on day 1 and there are already many piyyuṭim.