Is the default file appended of Logback:
ch.qos.logback.core.FileAppender
synchronous or asynchronous? It seems to be synchronous as the logs are being shown as part of same thread.
Yes, it's synchronous by default. You can see a config example on how to make it asynchronous in the documentation.
This way, you can make any Appender asynchronous much easier (by simply wrapping it in an AsyncAppender) than if all Appender implementations would have to manage the asynchronicity on their own.