IIS automatic redirect from primary to sub domain taking too long

I have an ecommerce style website I plan on replicating for multiple customers.  Some customers will have multiple subdivisons, each with their own subset of available assets, logins, etc.  To make this work we designed the backend SQL db to use a heirarchial structure (dbo.PrimaryDomains, dbo.SubDomains, dbo.MultipleTablesWithForiegnKeysToDBOSubDomains).  The issue I am having is replicating the site for a customer with only one domain.  The heirarchial windows folder and SQL db structure we use allows users to go to the upper level index.php file, select their subdivision and be redirected to their lower level index.php file.  For this new customer I have the upper level index.php file automatically redirecting the user to the lower level index.php, but for slower internet connections and/or slow computers the upper level index.php file is displaying for too long during the redirect.  

Is there a way in IIS to have the primary index.php redirect to the subdomain index.php without anyone noticing the redirect is occuring?  Or are there any other approaches I could take to avoid hitting on the primary index.php all together?

The web server folder structure of the site looks like this:
InetPub
  TheFirstSite
      Access
      Application
      ASPNet_Client
      SubDomainA
      SubDomainB
      SubDomainC,etc.
      CSS
      etc.etc.
  TheSecondSite
      Access
      Application
      ASPNet_Client
      SubDomainA
      CSS
      etc.etc.

The subdomains pull from the upper level folders the templates, style sheets, etc. that they all share and need and their unique files are located in their lower level folder(s).