Investors parking cash in the safest of U.S. assets probably won't see immediate relief from depressed returns when the Federal Reserve starts raising interest rates.

The possibility of as many as two rate increases by year-end has failed to outweigh a mismatch between supply and demand for the shortest and safest of debt, with Treasury bill rates maturing through October locked at about zero percent. Global regulatory changes are boosting the need for high-quality assets from bills to repurchase agreements to bank deposits just as the supply is sliding.

"As the Fed begins tightening, yields on all short-term instruments won't recalibrate higher," said Jerome Schneider, head of short-term strategies and money markets at Newport Beach, California-based Pacific Investment Management Co. "There is a misperception in the market that all asset yields will move higher. We are in a new paradigm and there will be these dislocations."