Performance issues on price set retrieval

I have hit a serious performance issue affecting 4.7.15 and, I believe a couple around this.

The issue is because the price set api is called from the contribution view page.

The price set api adds data about the price set using the getUsedFor function.

This function looks for forms that use the price set. If it does not find them then it looks at every line item linked with the price set. For the default price set this could mean hundreds of thousands of lines.

This was a pre-existing problem with the price-set api/ the code it interacts with exposed by a core change to use it.