ODP.NET

My application is very critical on timing, so it forks 15 threads to go to oracle 11g server to submit query to read data parallelly. Here comes the problem, 15 threads call oracle connection open at the same time, I config the pooling be Min Pool Size=15; Connection Timout=60. When I run select * from v$session right after 15 threads run and I notice the 15 connections in the pool for this application account created slowly, one by one. So if 15 thread open connection at the same time, it is very slow, the last connection need wait for 50 seconds.
How to config the connection pooling if multithread trying to open connection at the same time? I tried Incr Pool Size=15; same thing: the connections in the pool generated slowly one by one. Or connection pooling not fit for this situation? Can I keep pool in oracle exist forever so that only first time multithread run is slow?

Thanks,