Wilbur Ross, Donald Trump’s pick to head the Commerce Department, at Trump Tower in New York late last month.
Wilbur Ross, Donald Trump’s pick to head the Commerce Department, at Trump Tower in New…

The concentration of wealth among president-elect Donald Trump’s nominees is setting up an arduous and expensive Senate confirmation process that could slow implementation of the White House agenda.

The Republican has so far appointed five billionaires and half a dozen multimillionaires to serve in his administration. To win confirmation, they will be required to disclose their financial holdings and divest themselves of any assets that could present a potential conflict of interest with their new posts.

While the process may not hurt the nominees’ chances of winning Senate approval, it could eat time that prevents the Trump administration from hitting the ground running. For months after President Barack Obama took office in 2009, important posts in the Treasury department remained vacant—in the throes of the financial crisis—as candidates were vetted by the Senate.

Mr. Trump’s picks for his cabinet so far have a collective net worth of close to $10 billion, according to a Wall Street Journal analysis of Forbes magazine’s net-worth estimates and financial-disclosure data.

Ethics officials “have some experience with people with very complicated financial situations, but that’s not the norm,” said Steven Rattner, a multimillionaire who in 2009 spent $400,000 on legal fees to prepare for accepting a position in the Treasury department. “Most people in the government are not people with the kind of wealth of the Trump people.”

History suggests the president-elect’s team will have its work cut out. In 2008, Mr. Obama approached billionaire Penny Pritzker about joining his cabinet. She ultimately decided against it, in part because of the divestiture and disclosure process, according to a person familiar with her decision. She agreed to head the Commerce Department four years later, winning confirmation in mid-2013, six months after her preparations began.

In May 2013, a month ahead of her confirmation, Ms. Pritzker signed an ethics agreement outlining the steps she would take to avoid conflicts, including divesting herself of interests in 221 entities and resigning her positions at another 158 entities.

Even appointees who don’t require Senate confirmation are faced with a complex series of tasks.

“It’s a very painful process,” said Mr. Rattner, who began consulting lawyers in December 2008 about whether it would be feasible for him to join the Obama administration. He was formally appointed in February 2009.

“I don’t think this is government out to torture you,” Mr. Rattner said. “I think it’s government out to prevent conflicts of interest.”

He left his post in the administration after six months.

Henry Paulson, a Goldman Sachs Group Inc. chief executive worth about $127 million when President George W. Bush tapped him as treasury secretary in 2006, spent about as much on professional fees to comply with federal ethics and conflicts…