Problem picking multiple files from the receive location.

In my application there are big messages, so to make the processing proficient I am splitting the messages into pieces and dropping them to the receive location of an orchestration. The average size of the messages is around 300KB and there are around 200 messages, few of them gets picked up immediately, however some of the messages get picked up by the orchestration only after around 1 hour. Along with this application there is another application which runs in parallel using different host instance and has around 1000 messages which creates around 1000 orchestration instances. This process gets completed within 10 minutes.
Please see below my questions.

1. Is there any restriction on the number of instances created by an orchestration at a time in BizTalk? If so how can I handle this issue?
2. Do orchestration instances hold the memory even after completion? If so, how can we make sure, its cleaned up?
3. How can I force BizTalk to release the memory it holds as we can do in .NET?