Why does this "Slow network detected..." log appear in Chrome?

This means the network is slow, and Chrome is replacing a web font (loaded with a @font-face rule) with a local fallback.

By default, the text rendered with a web font is invisible until the font is downloaded (“flash of invisible text”). With this change, the user on a slow network could start reading right when the content is loaded instead of looking into the empty page for several seconds.

• Related Chrome issue: https://bugs.chromium.org/p/chromium/issues/detail?id=578029. (A change enabling this behavior for 3G connections landed in September; this should be the reason you got the message.)
• Related source code: https://chromium.googlesource.com/chromium/src/third_party/+/master/WebKit/Source/core/css/RemoteFontFaceSource.cpp#74

I faced same issue for chrome build 61.0.3163.100 on MacOs Sierra with localhost as server. Chrome started logging this message when I changed network speed configuration to 3G fast/ 3G slow and again back to Online.

Fix: When I tried selecting Offline mode and again Online mode, the logging issue disappeared. (This fix may no work on some devices or versions)

Update on 30th Jan 2018

I updated google chrome to Version 64.0.3282.119 (Official Build) (64-bit), it seems this bug is fixed now.