This is mine. The same thing happened to me, it slowed down my N964GB. I have about 15GB only of space used at the moment.

root: 893MB
system: 893MB
desktop: 200MB
background: 34MB
applications: 591MB
standby: 467MB
-
root -system: 0MB
system -desktop -background -applications: 66MB
applications -standby: 0MB
total: 893MB
total device memory: 985MB

--Usage
root: 813MB
system: 776MB
-
desktop: 98MB/100MB
background: 9MB/10MB
applications: 515MB/600MB
standby: 441MB/500MB
subtotal: 623MB/710MB

root -system: 37MB
system -desktop -background -applications: 152MB
applications -standby: 74MB

total: 813MB/985MB