multithread

Hello,I have written a program that is about serial COM.I created two thread in this program ,one received data from serial COM and insert them into a string list,another get string from this list and insert them into a database.The string list is a global variable.I expanded this program and now it can manage two serial COM.So there are four thread in this program .Every two thread manage a serial COM.And each serial COM has a string list in the program to store data.But now the rate of one serial COM is slower then another clearly.If I run the original program twice and ench deal with a serial COM, the rate of two serial COM is same almost.I must point that the flux of the two serial COM is every high.I don't know the reason of this problem and how to resolve . Please help me! Thank you every much!

[MoMoPi]

can you make you story short? I try to help you,but I don't understand the problem ?