Slowest many-one reduction?
Allender suggests the answer is no:


  There seems to be no pair of natural NP-complete problems A and B known, where a reduction from A to B is known to require more than linear time (even under the assumption that P $\ne$ NP)


Reference:

E. Allender and M. Koucký, Amplifying lower bounds by means of self-reducibility. Journal of the ACM 57, 3, Article 14 (March 2010).