lift adobe premiere ram allocation limit?
Premiere does not have a maximum memory usability limit. As several others pointed out, this is probably a motherboard limitation issue. @nofare's solution for working with proxies is one way to reduce the memory needs for your project. 

My guess would be, based on your original statement, that the problem is not memory allocation, but disk read speed or perhaps some overly-gargantuan source files. I edit HD and 4K in Premiere on a MBP with 16GB ram all the time and can't ever remember maxing out my RAM. 

A few other things to check, assuming for the sake of argument that your motherboard is not the problem:


What is Premiere's internal "soft" maximum memory allocation set to? This will not only tell you how much your OS is actually seeing, but also how much Premiere is willing to consume. 
Where are your source files located - on internal storage, a USB drive, in the cloud, etc? Premiere could be slowing down as a result of trying to read your source files, particularly if you are using multiple streams simultaneously, from slow I/O sources like USB 2.0 or cloud-based drives.
What is your largest-resolution timeline asset? And, how is it encoded? If you're using gigantic Photoshop files on your timeline, yeah, well...it's gonna be slow. Same goes for RAW video or even 4k mpeg files. They just don't edit easily and could contribute to choppiness or temporary "freezing" in the interface.
What is your playback display quality set to? I typically work with this set to 1/2 or even 1/4 for optimum fps.