FPS making things behave differently

Hi guys, I have a loading circle made from a material, a value called ‘percent’ controls how much progress fill it has. (fairly straight forward). Its a circular loading bar essentially, but I am using it for a reaction timer type thing.

I created a blueprint that has a simple variable to control the fill percent, the player presses space bar and the bar starts to fill.

The problem- When my PC is in power-saver, the bar fills a lot slower than if it was in balanced mode.
It’s simply: fillProgress += deltaTime right?