# in app.psgi
format => "%v %h %l %u %t \"%r\" %>s %b %D";
DESCRIPTIONPlack::Middleware::AccessLog::Timed is a subclass of Plack::Middleware::AccessLog but uses a wrapped body handle to get the actual response body size %b (even if it's not a chunk of array or a real filehandle) and the time taken to serve the request: %T or %D.
This wraps the response body output stream to capture the time taken for the PSGI server to read the whole response body.
This would mean, if the middleware is in use, it will prevent some server-side optimizations like sendfile(2) from working, as well as middleware like Plack::Middleware::ContentLength can't guess the body size out of the file handle.
If all you want is to capture the time taken in your PSGI application and do not want the wrapped body behavior described above, consider instead applying Plack::Middleware::Runtime and using Plack::Middleware::AccessLog to log the "X-Runtime" header.