The phrase "determinative factor" is used to describe an influential or decisive element that has the power to strongly affect an outcome or decision. It refers to the key factor that ultimately determines or settles something. Full definition
In Canada, Hubbard and company explain, the degree to which information is treated as confidential is important, but not as determinative a factor in assessing the reasonableness of a privacy claim. (slaw.ca)
In this instance, if site speed becomes a large determinative factor to rankings, the end - user may also lose out, if they ultimately would prefer a slightly slower site but more relevant information. (searchmarketingstandard.com)