CAUTION: SLIPPERY FLOORThe Arduino Library Manager installs the ArduinoJson version 6 by default.
However, using version 5 is highly recommended because version 6 is still in beta stage.
Open the Arduino Library Manager and make sure that ArduinoJson version 5 is installed.
First of all, ArduinoJson is not slow by itself. It’s slow when used in conjunction with the
WifiClient from the ESP8266 core.
The problem is that there is no buffer between ArduinoJson and the WifiClient.
To solve this, either: