Without seeing the logs it can be difficult to determine where the delay occurs. Here are a few performance thoughts you might try.
When a client is first provisioned, the initial evaluation can take a considerable amount of time, increasing due to the number of subscribed sites, how much content is in those sites, the number of operator accounts with rights, and any content in those operator sites.
In general, make sure your Master Action Site has as little content as possible.
To speed up initial provisioning, you might create a new Custom Site for post-image provisioning. That site could contain only the SWD baselines that you want to apply. You don’t need a copy of your source fixlets in this site, just the Baseline containing the fixlets/tasks. On your other sites, you could put in a computer subscription delay as jgstew describes at Delay BigFix site subscription to speed up client provisioning
If you have a large number of Operator Accounts, you might also delay the operator rights assignment by using a Custom Property, similar to the Site Subscription delay. For instance you could make a Custom Property for “NewProvisionedComputer” with Relevance of
not exists absolute values whose(it > 2*hour) of (now - it) of minima of subscribe times of sites
As part of your Operator Rights assignments, you could configure “normal” operators to only have rights when this value is False, and set up a dedicated operator account for newly-provisioned computers that has rights when this is set to True. Your SWD baseline tasks could be actioned by this dedicated operator account, so the client will start downloading/installing the SWD baselines before it subscribes & evaluates your normal operator account content.
You can also speed up the initial evaluation by setting __BESClient_Resource_StartupNormalSpeed as described at https://www.ibm.com/developerworks/community/wikis/home?lang=en#!/wiki/Tivoli%20Endpoint%20Manager/page/Configuration%20Settings
Are you using any bandwidth throttling at the relays that might slow downloads? Have you distributed relays to reduce any network delays?