I’ve been training consistently with a Tacx Vortex Smart for the last year and I have always felt that the beginning of intervals felt significantly harder than the rest. I had always put this down to my body having to readjust to the increased work.
I have only very recently acquired a power meter in the form of a Garmin Vector 2S and - lo and behold - the power during the first 4/5 minutes is significantly higher than the target power reported by the trainer! (As can be seen in the graph, ignoring the lack of synchronisation due to Garmin and TrainerRoad dealing with pauses differently).
The power meter tracks 10/15 W higher, on average, during steady-state and that seems perfectly reasonable given the loss of power between pedals and wheel.
I should add that this is in ERG mode, as I never switch to Resistance mode. Is this normal? Is it something that other Vortex users are facing? Is it something that would happen with any trainer?