If you could quickly translate a lambda-calculus expression into a program written in some other model of computation (that was easier to simulate on a Turing machine) and back, then you could do the complete reduction in the other model, only using lambda-calculus as your input and output. To perform the entire reduction you would still have to scan the expression in O(n), but it wouldn't necessarily be O(n * number of reduction steps). There could potentially be a relationship between the number of beta reductions and the number of steps required to execute the reduction in the other model, in which case beta reduction count would be justified as a measure of time.