session swapping on multiple request at the same time

I am having an ecommerce website running in codeigniter. The sessions are working fine on normal case. The problem starts when, say for example if two users are there on my website and assume that they are logged in, and when they click any action like add to cart or proceed to checkout or place the order the same time the login sessions are swapped.
Could anyone please help me on this! I have searched a lot on this but couldn't find a good solution.
I am using the config file like this

Code:

$config['sess_driver'] = 'files';
$config['sess_cookie_name'] = 'ci_session';
$config['sess_expiration'] = 7200;
$config['sess_encrypt_cookie']  = TRUE;
$config['sess_save_path'] = sys_get_temp_dir();
$config['sess_match_ip'] = TRUE;
$config['sess_time_to_update'] = 300;
$config['sess_regenerate_destroy'] = FALSE;

I won;t be able to use the sess_driver as database since it will affect my website performance i believe.

PS: The "Two" users connecting from different computers with different IP's.. basically this is happening when the "Two" users request hit the server at the same time.

Thanks in advance!.