Google Chrome	67.0.3363.0 (Official Build) canary (64-bit)
Firmware Version	Google_Samus.6300.276.0

For now, go to Chrome://flags > Enable Experimental accessibility features to get the latest version of STS. In the future, this step won't be needed.

A user reported lagging/delayed/stopped speech from STS under heavy use with both modes. 

Steps to repro:
# Enable STS in Accessibility menu or in Accessibility Settings. Note, it's not recommended to use with ChromeVox so please use this as the only active screen reader 
# Load a page with a lot of text, for example, a long email in Gmail
A tip for getting lots of text would be to zoom out of the window using ctrl + shift + minus to get more content onscreen. 
# Read over a lot of text many times in a row, alternating the two invocation methods:
highlighting text and then pressing search + s
holding down the search button and using the mouse to draw a rectangle around text

I did 5 sets of 4 alternating uses of the feature to simulate heavy use. That is, I'd use the mouse, then the keyboard, then the mouse, then the keyboard in rapid succession. I'd then repeat that set of 4 actions 5 times in a row.

Expected: text always read smoothly
Actual: text read smoothly as long as ctrl is pressed to silence speech between switching modes or between using search + s twice. 

This indicates that STS under stress is perceived to be ok but there is other bug behavior that seemed to indicate instability.