RegFileExport

RegFileExport is a speedy and portable command-line tool that gives you the possibility to export the entire registry to file as a backup measure, as well as to extract specific keys and subkeys.
It works with registry files located on another hard drive and, in some cases, it's also capable of processing data from corrupt registry files.

How it works

The usage is RegFileExport source_registry_file destination_registry file registry_key, where you can indicate the registry file you want to create a copy of, along with the name of the new file. If they're in different directories than this program, make sure to write their paths too.
The registry_key is optional. If specified, the app is instructed to extract only that key together with all containing subkeys, in order to save them to file. The destination_registry may also be left unfilled, which means that the new file with be created in the standard output.
Several examples are listed in the console window when calling RegFileExport's process without any arguments. Moreover, you can check out a help manual in the downloaded package.

Portable utility

Since there's no installation involved, you can save the downloaded .exe file in a preferred directory on the disk or copy it to a USB flash drive in order to easily run it on any machine without setup. It doesn't need DLLs or other components to run, create files on the disk without your knowledge, or add new entries to the registry.

Evaluation and conclusion

It executed commands rapidly in our testing while remaining light on system resources consumption, using low CPU and RAM. No error dialogs popped up and it didn't hang or crash.
All in all, RegFileExport delivers a fast and effective method for backing up the entire registry or just specific keys. Plus, it's free.