Rashi and Torah Temimah, from Yevamos 47a-b: If one comes to convert nowadays, we ask, why do you want to convert? Don't you know that Yisrael are depressed, pushed around, humbled and afflicted?! We inform him of the punishment of Mitzvos. We do not [frighten him] too much, nor are we so exacting with him - "va'Tere Ki Mis'ametzes? va'Techdal le'Daber Eleha."

Vilna Gaon: The Yetzer is like a fly. It sits between two chambers of the heart. The Yetzer Tov counsels only to do Mitzvos. The Yetzer ha'Ra overpowers him every day (Sukah 52a), and is renewed every day (Kidushin 30b). If it cannot make him do an Aveirah, it entices him to a Mitzvah, and wraps it with an Aveirah in the Yetzer's way. To test from which Tzor a Mitzvah came, if when doing it his limbs move with zeal, presumably, it is from the Yetzer ha'Ra. The limbs are heavy, from Yesod he'Afar. Their nature is to pursue desires of the body, which descends. If they are filled with desire, it must be is the Yetzer ha'Ra's counsel, to trap it! If his limbs are heavy, it is the Yetzer Tov's counsel; the Yetzer ha'Ra seizes the limbs and tries to stop them. We find that Reish Lakish was able to jump across the Yarden for the sake of an Aveirah. After he accepted the yoke of Mitzvos (the Yetzer Tov), he could not jump back (Bava Metzi'a 84a), for the Yetzer ha'Ra stopped him. Also here, Na'ami saw that Rus needed to exert to follow her, even though Rus was younger. This showed that the Yetzer Tov motivated her, so Na'ami ceased trying to dissuade her.

Malbim: Acting with continued exertion showed that she wants to convert with all her heart, so she ceased trying to dissuade her.