Infrared is not only about air temperature
Infrared panels radiate energy toward the body, so some manufacturers allow sessions before the display reaches its maximum setting. Follow the model instructions and allow the emitters to stabilize rather than chasing a generic thermometer number.
Cold rooms add time
An unheated garage or basement increases heat loss into the air, glass, and wood. If winter warm-up becomes excessive, confirm the model is approved for that ambient temperature before trying to insulate or modify the cabin.
When slow warm-up may signal a problem
Compare performance under similar conditions. A sudden change can point to a door not closing, blocked or changed vent position, a disconnected heater panel, low supply voltage, or a control issue. Do not open electrical enclosures; use manufacturer troubleshooting steps.
Sources and documentation
Specifications, claims, and installation requirements can change. Confirm the current manufacturer manual for the exact model and variant.