Hi Suwandi,
Have you checked the IDoc partner profile setting in ECC based on Prasanthi's suggestion above?
Basically for inbound IDocs into ECC, if it is a high volume interface, the corresponding partner profile should be set to "Processing in background".
For your case, it might have been set to "Immediate processing" - with this setting, when the IDoc is sent from PI, the LUW session both creates the IDoc and tries to process it to create the target ECC document. The first step is very quick but the latter step can take up quite some time. This then might hold up the connection on PI side.
When the partner profile is set to "background", only the IDoc is created in the LUW session and then it is closed - this happens quite quickly. On the ECC side, you then have to set a background job to run program RBDAPP01 with the correct variant to process the IDocs created.
OSS note 1333417 (Performance problems when processing IDocs immediately) explains the situation in more detail.
Rgds
Eng Swee