EMS skips first 10-15s of each recording

For the following setup, we face a delay of about 15s from the issue of the "record" command until the actual recording happens.

• The RTMP stream is already present on EMS (another client views the stream)
• Issue ‘record’ command, recording to an MP4 file on the local disk where the EMS is running. EMS returns the ID of the recording
• The recording exists in ‘liststreams’, but the content of the recording file has a delay of 10-15s. This means, that the first 10-15s are not included in the recording.

What causes this delay? I cannot see any error in the logfile. How can we speed up the start of the recording, such that it starts instantly? I cannot find anything of matter in the documentation.