Much faster PHP with FastCGI.

FastCGI for PHP makes all your PHP applications run through mod_fastcgi instead of mod_phpsusexec. FastCGI applications are fast because they’re persistent. There is no per-request startup and initialization overhead. This makes possible the development of applications which would otherwise be impractical within the CGI paradigm (e.g. a huge PHP script, or an application which requires a connection to one or more databases).

Advantages :

• PHP scripts will run faster. The PHP interpreter is loaded into memory rather than calling from storage for every hit, greatly improving performance of your scripted site.
• You will use less Server Resources. Since the server does not have load the PHP interpreter for each hit, you will be able to accommodate a higher traffic site without exceeding your CPU quota.
• NO modifications to your existing code are required. Everything you currently run will work with FastCGI for PHP.

Potential Problem / Limitation :

• You will only have one php.ini file available for all subdirectories (e.g. /home/USERNAME/public_html/php.ini). This is necessary to help optimize the website code as much as possible. If you need multiple php.ini files to accommodate different scripting needs, you can disable FastCGI for PHP on any of your subdirectories while leaving the rest of the account enabled for performance. You may have to contact support if you need to do this.
• There can be a few minute delay in any updates you make to your PHP environment (ie, php.ini changes). Since your php.ini is loaded into memory for greater speed, every hit does not re-read it from storage.

Not all PHP host offer this service, but BlueHost and Hostmonster does offer PHP 5 FastCGI feature.

Related :

Understanding FastCGI Application Performance.