Is there a consensus on how much CTR you're giving up with context-based (as opposed to tracking-based) ad selection? Are we talking 1% or are we talking 50%?

Is there a consensus on how much CTR you're giving up with context-based (as opposed to tracking-based) ad selection? Are we talking 1% or are we talking 50%? (What if tracking-based ads are actually less effective, as in: "I'm hesitant to click on an ad that keeps following me around"; "I've already bought this thing, will you please stop showing me this ad?"; etc)