SURF description faster with FAST detection?

Question

for my master thesis, i am running some test on the SIFT SURF en FAST algoritms for logo detection on smartphones.

when i simply time the detection, description en match

Answer 1

AFAIK the most time consuming part of SURF descriptor extraction is subpixel extraction of patch having (2.8*keypoint.size x 2.8*keypoint.size) size around the every keypoint.

So here is my guess: keypoints found by FAST detector always have their size equal to 7 but SURF detector can find keypoints of much bigger size. So 180 "big" keypoints are processed longer than 319 "small".