When you have an import script that takes long to execute, the browser seem to lock up and you cannot access the website anymore. this is because a request is reading and locking the session file to prevent corruption.
you can either 
- use a different session handler with session_set_save_handler()
- use session_write_close() in the import script as soon you don't need session anymore (best moment is just before the long during part takes place), you can session_start when ever you want and as many times you like if your import script requires session variables changed. 
example
<?php
session_start(); //initiate / open session
$_SESSION['count'] = 0; // store something in the session
session_write_close(); //now close it, 
# from here every other script can be run (and makes it seem like multitasking)
for($i=0; $i<=100; $i++){ //do 100 cycles
 session_start(); //open the session again for editing a variable
 $_SESSION['count'] += 1; //change variable
 session_write_close(); //now close the session again!
 sleep(2); //every cycle sleep two seconds, or do a heavy task
}
?>