ResponseTimeAttribute returns time from before the response ends when used in expression filter

When response time attribute is used in expression filter e.g. like this:

header(header='resp-time', value='%{RESPONSE_TIME}');

It seems to work correctly when we request for welcome-content which is maintained by the file-handler. But in case we perform request against servlet (attached) then the time returned in the header resp-time does not match expectations as it is very close to 0. Is it by design due to some restrictions for servlets or is it a bug? There is no documentation to refer to or I did not find any.