Showing on console: Slow network is detected. Fallback font will be used while loading
Setting the following chrome flag to Unknown will prevent that message appearing on dev-console :
chrome://flags/#force-effective-connection-type
ᴛᴇꜱᴛᴇᴅ ᴏɴ
Google Chrome
Version 63.0.3239.84
Same issue in localhost server.
But if you don't want to see these messages in log:
On the chrome tools => console settings => checked on User messages only
Simply add font-display to all css font-face definitions to remove error from console.
@font-face { font-family: ExampleFont; src: url(/path/to/fonts/examplefont.woff) format('woff'), url(/path/to/fonts/examplefont.eot) format('eot'); font-weight: 400; font-style: normal; font-display: block;}