Building histograms [How to prevent Photoshop from continually building histograms slowly?]

Choose the Histogram layer. In the upper right, click on the 4-line symbol. and choose Close.

Are you using the histograms? If not, just close that window. If you are using them and would like it to be more active, change it's location so that it's always visible. that way it doesn't have to recreate it every time you open it.