[sqlite] The problem of inserting data too slow with index

[aa]

Hello, 
       I am a sqlite user from asia. I use sqlite for more than a year , now 
the version i am using is 3.8.11.1.
       But recently I meet a problem about inserting data into table.It is too 
slow whit index.
       The first I create a table like this:
        CREATE TABLE mac_tb  (mac BIGINT PRIMARY KEY?
        If I insert into mac_tb with mac ordey by num desc or asc , then the 
speed is fast.
        If I insert the mac randomly, then the speed will grow slower whit the 
increasing of data. It become slower if the count of mac_tb
more than 2,000,000.
        But if i create another table without index, than the speed is so fast.
        I don't know why the performance difference is so great.