That's why. You almost filled it up. It's good to leave some empty space. Some times the iPhone's RAM fills up and uses the space on the NAND flash as Page-outs.
So say your iPhone is filled up, it will slow down when it gooble down all available RAM.
However, as iphone.aggie said before, the best non-restore solution is reset everything (don't erase content). But for best results a Restore and setting up as new is always a plus.