Tension is good. It makes the reader turn the pages. However, constant high tension soon gets dull. The readers can’t sustain continuous scared excitement, and after a while, instead of roused, they become bored.
It’s like the waves on a stormy sea: the peaks are only high because of the troughs between them. If there were only continuous peaks without any troughs, the sea would be flat.
Your job as writer is to create not just the peaks, but the troughs which make the peaks look high.