Sharepoint - Immediate Redirect page in sharepoint
It could be achieved with HTTP Redirect in IIS using IIS URL Rewrite module. This module allows to create various rule actions including redirect and request abort.
For your scenario i recommend this option because this should redirect the request faster as the redirection happens as soon as the HTTP request received by IIS.
Configuration
Let's describe how to configure Rule that redirect a sites homepage to a subsites homepage in URL Rewrite.
For example to redirect request from:
http://{ServerName}/teamsite/SitePages/Home.aspx
to:
http://{ServerName}/teamsite/itdepteamsite/SitePages/Home.aspx
the following Rule is used
,where
Pattern: ^(.*/)?teamsite/SitePages/home.aspx$
Redirect URL: teamsite/itdepteamsite/SitePages/Home.aspx
You can use a standard HTML redirect with a wait period of 0:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<html>
<head>
<title>Redirecting...</title>
<meta http-equiv="REFRESH" content="0;url=http://www.yoururl.com">
</head>
<body>
</body>
</html>
Changing the value of x in content="x;url" determines the number of seconds to wait before redirecting.
See: Redirect default.aspx or change default landing to page in _layouts folder?
and Change default site in Sharepoint foundation
Perhaps you can change the Welcome page to subsite homepage. To do this Go to Site Settings > Look and Feel -> Welcome Page