The Premiere Pro 5.5 upgrade sports a number of fresh features. It has an improved Mercury Playback engine; better DLSR editing, specifically the merge clip feature connects video clips to sound clips when shooting using a double system acquisition; customizable keyboard shortcuts so you can make your keyboard emulate Avid Media Composer or Final Cut Pro 7; an enhanced insert and overwrite editing features; and a new importer allows you to natively work with Red Epic footage. There are more GPU-accelerated features, including time remapping, speed change, footage interpretation options, field order processing, filmic transitions, blur effects and more. The existing support for Red R3D is enhanced and now Epic footage can be imported. What is significantly missing is native support for Arri Alexa footage.
What is called the Mercury Playback Engine is a collection of features that includes 64-bit addressing, enabled multithreading and the ability to access CUDA technology on specific Nvidia cards. This is specialized functionality built into Premiere Pro 5.0 that has been enhanced in PP 5.5 enabling it to do a lot more. As a 64-bit app, Premiere allows for all the RAM in a system to be addressable. This, coupled with the reasonably stable 64-bit Windows 7 operating system, makes for a mostly reliable editing environment.