This study investigates undergraduate students’ English reading proficiency through a sequential mixed-methods design that integrates neurocognitive evidence, behavioural performance, learner perceptions, and background characteristics within a computerised assessment context. In Phase One, paper-based testing (PBT) and computer-based testing (CBT) were compared using electroencephalography (EEG) and qualitative inquiry. Five Thai EFL undergraduates completed parallel Thai Reading Evaluation and Decoding System (Thai- READS) tasks in both formats while EEG data were recorded using a single-channel MindWave Mobile 2 device. Wilcoxon signed-rank tests indicated no significant differences in READS scores between PBT and CBT (Z=−1.633, p=.102). EEG analyses revealed largely comparable cognitive processing across test modes; however, a significant difference in alpha frequency emerged for difficult items answered incorrectly (Z=−2.023, p=.043), with higher alpha frequency observed in the CBT condition, suggesting differential neural engagement under increased task difficulty. Semi-structured interviews further indicated that students generally perceived CBT as convenient, organised, and supportive of focus, while also expressing concerns related to technical reliability, digital navigation, and limited opportunities for annotation. Phase Two analysed Thai-READS performance data from 3,274 undergraduates using a two-way analysis of variance to examine the effects of academic achievement (GPA) and socioeconomic status (SES). Results revealed significant main effects of both GPA and SES on reading proficiency (p<.05), with GPA demonstrating a larger effect size (η²=.06). The findings support the feasibility of CBT while highlighting the importance of considering cognitive processing and learner background to ensure valid and equitable assessment practices in Thai higher education.