Urban trees face increasing extreme heat, yet the timing of their short-term physiological responses remains unclear. This study investigated thermal stress dynamics in Acer campestre during two contrasting UK summers (2022 and 2023) and examined how sampling frequency affects the detection of temporal lag effects. Chlorophyll fluorescence parameters (NPQ and ETRmax) were measured daily during the 2022 high heat days and weekly under the milder conditions of 2023. Generalised linear mixed models and cross-correlation analyses explored the relationships between leaf temperature and photosynthetic performance. In 2022, NPQ showed increased morning thermal sensitivity under extreme heat, but detrended analyses did not reveal strong multi-day lag effects. ETRmax responded in synchrony with temperature, showing no detectable time lag. In 2023, apparent multi-day correlations matched exactly with the weekly sampling interval and were diminished or nullified by detrending, suggesting sampling artefacts rather than physiological memory. Across both years, morning measurements consistently indicated the greatest thermal sensitivity. These findings reveal that Acer campestre exhibits threshold-dependent stress responses and highlight the importance of high-frequency sampling for accurately capturing short-term physiological dynamics. Sparse sampling can produce misleading lag patterns, emphasising the need to match measurement frequency with the biological response timescales in urban tree monitoring.