This study explores the nonlinear relationship between text length and oral reading speed among Japanese language learners from non-kanji backgrounds. Over four years, 126 university students read texts ranging from 50 to 700 characters. Reading speed (characters per second) showed a distinctive non-monotonic pattern: it decreased up to approximately 300 characters, increased between 300 and 600 characters, and declined again beyond 600. These findings suggest that moderately long texts facilitate fluent reading by supporting contextual prediction and rhythm formation, whereas very short or overly long texts disrupt processing due to fragmentation or cognitive overload. A comparison between two reading formats—whole-text reading (Condition A) and paragraph-segmented reading (Condition B)—revealed that Condition A generally led to faster reading speeds in the 300–600 characters range, highlighting the importance of cohesive rhythm and uninterrupted flow. Crucially, the nonlinear trend observed may reflect not only surface-level behavior but also deeper neurocognitive dynamics. Studies in cognitive neuroscience suggest that oral reading engages distributed brain regions and that reading rhythm and fluency are associated with synchronized brain activity. Therefore, this study proposes the need for future research using EEG to investigate real-time neural processing during oral reading. The findings have pedagogical implications for optimizing reading materials and training methods. They also open new directions for evidence-based language education supported by neuroscience, particularly in identifying optimal reading conditions that enhance both performance and cognitive efficiency.