If your primary has huge load or if there are huge gap on your standby, you may need to increase your apply rate on your standby database.
Do not forget your standby server’s CPU and Disk should be strong enough for this actions.

Here is the steps

1. First let us check current gap:

2. Check related parameter on Standby database

SQL> show parameter PARALLEL_EXECUTION_MESSAGE_SIZE 

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
parallel_execution_message_size      integer     16384

Let us change parameter

SQL> alter system set parallel_execution_message_size=32680 scope=spfile sid='*'; << reboot standby database

4. Start parallel recovery on stabdby database

SQL> alter database recover managed standby database using current logfile disconnect from session parallel 64;

5. Check current gap:

Reference:

DATA GUARD REDO APPLY AND MEDIA RECOVERY BEST PRACTICES
http://www.oracle.com/technetwork/database/features/availability/maa-wp-10gr2-recoverybestpractices-131010.pdf