You will NOT gain performance in any way. You will maintain performance and current speed of your PC by keeping your registry clean and free of unused keys. If you are at a point where your PC hasn’t been maintained for a few years and has slowed down considerably it is time for a complete reinstallation of the operating systems and all programs. There isn’t a program out there that will fix you PC. You have to maintain it properly.
It will not even do that. The Registry is not and was not the cause of the slow down. The Registry is a highly optimized hierarchical database it can handle millions upon millions of entries without ever slowing down. Tackle the problem at the source, the applications the user installs. Cleaning the Registry is only going to bring grief and instability with nothing to gain.
Yes, Windows registry is “supposed” to handle millions of registry keys, but what happens after tons of programs have been installed and uninstalled over the course of a year? They leave remnants in the registry which in turn WILL slow your PC;s speed down. I do NOT recommend anyone to go into their registry and start editing and deleting keys on their own. Of course this would bring devastation to a stable system and cause multiple errors, depending what was deleted or edited.
Keeping a clean registry with the use of a program like CCleaner or comparable and trusted WILL help maintain your PC’s performance and speed.
The manual editing in registry files , without any deep knowledge can create problem for the user. The removal any useful file can leave behind a corrupted form of data.
PC Tools Registry Mechanic
Mil Shield
I’m running these two on Windows 7 systems now and my systems are faster. I like Mil Shield custom cleaning for the whole Registry and Windows. This function is smart to me because shows first what will be cleaned and then you can decide what to be cleaned.
PS
I want to add that I love kangaroos too 
Regcure, Registry Mechanic, Advanced Registry Optimizer
Registry Mechanic and Advanced Registry Optimizer, I have tested them…give good results but I want more.Now I’m trying Mil Shield.For now has a good Registry editor, no need to go in Windows Registry itself to edit keys/values.