DSO is also known as mod_php. DSO stands for: Dynamic Shared Object. This is an older configuration but is generally considered the fastest handler. It runs PHP as an Apache module. This means that PHP scripts will run as the Apache user, which is the user: ‘nobody’.
DSO has two drawbacks. First, all files created by a PHP script will have the ownership of ‘nobody’. They will not be readable from the web. Websites that need to upload files through PHP will run into file permission issues. This is common with WordPress users that upload files through the WordPress interface or utilize the auto-update feature. These will fail with DSO.
The second drawback is a security issue. Created files will have the ‘nobody’ ownership. If a hacker finds an exploit in your PHP script, they could implement a file that has the same privileges as important system files that are also owned by ‘nobody’. This will give them the ability to modify files outside of that user’s account. This is really bad for anyone who does reselling or simply is hosting other person’s sites. You would not one user to be able to affect another user. However, if there is only one account on the server (or if all the accounts are yours), then DSO may be right for you. The speeds benefits of DSO are unquestionable.
FCGI - FastCGI (mod_fcgid)
FastCGI is good at reducing CPU usage by leveraging your server's available RAM to keep PHP scripts in memory instead of having to start up a separate PHP process for each and every PHP request.
The main benefit of using the FastCGI handler is that allow PHP scripts to be executed by the actual user of the PHP script instead of using the Apache "nobody" user. FastCGI also doesn't require a single PHP process execution per request like suPHP does, so it can be much faster and reduces CPU usage by holding PHP scripts in memory.
The main disadvantage of the FastCGI handler is that it can be very memory intensive. This is because FastCGI keeps PHP sessions opened in the background in memory for quicker access and the ability to use PHP opcode caching can add to the memory usage as well. Additionally FastCGI can encounter an array of errors depending on how your PHP scripts are coded.