This is the same with the U.S. TTS voice.

I'm no longer able to repro this. Can you let us know if you're still noticing this behavior? 

Repros for me with:
54.0.2837.0
WHile adjusting to 100%, it sounds like it's increasing, but try navigating some text using 100% speech rate, and you'll observe it's the same as 71%. 

This still appears for me in latest canary.

Repros in: 55.0.2878.0

Related to pitch issues: 796238

Repros for me on Samus in Google Chrome	67.0.3363.0 (Official Build) canary (64-bit) Speech at 71% sounds very similar to me compared to 100%.

ChromeVox + Shift + Close Bracket = Decrease pitch of speech
ChromeVox + Shift + Open Bracket = Decrease rate of speech
ChromeVox + Close Bracket = Increase pitch of speech
ChromeVox + Open Bracket = Increase rate of speech

Test to show this: 
# Enable ChromeVox with ctrl + alt + z
# Fill a Text program document with one page worth of contents
# Set speech rate
# Start continuous reading with ChromeVox + r
# Use stopwatch to time how long it takes to read the whole page 
# Repeat on same content with the other speech rate

71%: 1 min 24 seconds = 84 seconds
100%: 1 min 17 seconds = 77 seconds 

Considering variability in human reaction time, the slower rate is only about 10% slower. 

CCing katie@ because it might have implications on the STS speed changes in https://bugs.chromium.org/p/chromium/issues/detail?id=819306