System Information
windows 7
tested on gtx 460 / gtx 970 /gtx 980 ti / gtx titan

Blender Version
Broken: 2.78

Short description of error
when topology change between frame because of a keyframe with a float time value, cycles is slow as hell

moving the shapekey key on frame 8 instead of 8.55 solve the problem

from 5 minutes to 4 hours on full scene.

Attached a file with the problem,

Exact steps for others to reproduce the error
open the file, render the frame 97, it's fast, render frame 96 it's slow as hell
on the full scene, it went from 5 minutes to 1 hours
Tested on CPU and GPU

i{F373749}