Each of the alternatives you offer present small errors.
The concept which underlies them is Decisions which are related to performance.
The first version employs a very common rule of English syntax to transform the phrase “related to performance” into “performance related”. This is how you would say it; but in writing it is conventional to join the two words with a hyphen, so the reader understands that the two words constitute one phrase: performance-related.
Unfortunately, this hyphen is often omitted, particularly in business use, which makes the reader's job a little harder.
In your second version, the one you think is the “right way”, you drop which are. This is perfectly acceptable and ordinary. However, you also drop the to, which is not acceptable, since the to defines the relationship between performance and related. Without the to, the reader seeing decisions related performance assumes that he is looking at a phrase of the first type, and interprets what you have written as “performance which is related to decisions”—not what you meant at all.
Either of these constructions is acceptable and unambiguous:
This whitepaper is intended as a resource to help you make performance-related decisions...
This whitepaper is intended as a resource to help you make decisions related to performance...