This study aims to investigate experienced Thai teachers' experiences of burnout during online teaching and learning, and examine how teachers' self-efficacy and burnout levels impact their teaching performance. The research concerns differences in perceptions of emotional exhaustion, depersonalization, personal accomplishment/assessment, teachers' self-efficacy, and online teaching performance among teachers in Thailand. The sample selection was conducted using a stratified random sampling technique. Data collection involved self-reported surveys from 243 elementary, secondary, and vocational schoolteachers in metropolitan areas and Thailand's north, northeast, east, and south regions. MANOVA and correlation analysis were employed to analyze burnout, teachers' self-efficacy, and teaching performance. The results indicated differences in online teaching performance and teachers' self-efficacy between two groups: high-risk and moderate-to-low risk of burnout. Teachers with a low risk of burnout demonstrated higher self-efficacy and better performance during online teaching. The study identified two burnout subscales--emotional exhaustion and depersonalization--originally included in the Maslach's burnout inventory. However, we also incorporated teachers' online teaching performance into the assessment, necessitating modifying the Maslach Burnout Inventory. Regarding implications, we recommend practical applications in policy improvements related to teachers' mental support and reducing burnout causes while enhancing online teaching performance.