On Thursday, one of JC Penney's biggest investors, hedge fund manager Bill Ackman, sent a letter to the company's board demanding that it speed up the process of finding a new CEO, adding that he's "very concerned" about the department store's future.

The letter came amidst a time of pain for the retailer. In April, after a year of terrible sales and a falling share price, the board abruptly fired CEO Ron Johnson, whom Ackman had fought to install, and brought former CEO Mike Ullman on to run the company in the interim. Since then, Ullman has overseen a handful of changes, such as reviving steep discounts to lure back core customers who had been alienated by Johnson's attempt to drop JC Penney's focus on coupons and blockbuster sales. But sales have continued to languish.

In July, the board agreed to begin its search for a new CEO in earnest, agreeing that it would aim to pick a new one in six months, sources told The Wall Street Journal. But Ackman insisted that was far too slow — and decided to air his view.

"I am very concerned about the future of J.C. Penney. While I supported the decision to bring back Mike as an interim CEO, it was based on Tom's statement that we would immediately launch a search process for a long-term CEO," Ackmann wrote in a publicly released letter, referring to Chairman Thomas Engibous. "Considering the scale of J.C. Penney, the seriousness of the issues it faces, and the complexity of its business, there are only a handful of executives with sufficient talent and experience to take on the CEO role."

Engibous wasted no time defending Ullman. "The Board of Directors strongly disagrees with Mr. Ackman and is extremely disappointed that his letter was released to the media at the same time that it was sent to the Board," he said in a statement. "Mr. Ackman has been integrally involved in the Board's activities since he joined two years ago. This includes leading a campaign to appoint the Company's previous CEO, under whose leadership performance deteriorated precipitously. His latest actions are disruptive and counterproductive at an important stage in the Company's recovery."

But does Ackman have a point? Ackman loves moving fast, says Laura Heller at Forbes. But it's this "very insistence on speed that got Johnson into so much trouble":

The changes he made, many of them inspired and necessary, were done with such speed that no tests were done, shoppers were ignored and employee trust was trampled on.

The mission was to do this fast, to make changes, to change Penney into a new kind of retailer.

It was to happen so fast it would make our heads spin. Instead, it failed so fast business schools will be teaching this cautionary tale for decades to come. [Forbes]

And it appears many are losing patience with Ackman. "Mike is working tirelessly to save this company, and it is despicable of Ackman to leak a letter asking for his removal," Howard Schultz, the chief executive of Starbucks, told The Wall Street Journal. "The irony is that Ackman himself has every step of the way severely damaged this company."

Friday, Ackman released a new letter, this time saying he has "lost faith" in Engibous as chair. The Journal published the note:

A proper functioning board needs to be fully informed about all material facts about a corporation in order to make deliberate and intelligent decisions. Extreme candor among directors is critical. Directors need to hear from one another in an open forum so all issues can be aired in a transparent fashion. Directors must put personal relationships and issues aside that might color their decision-making process. The board must be led by a Chairman who is unbiased, can make decisions without regard to personal relationships, and focused only on what is best for the corporation.

In recent weeks, our board has ceased to function effectively. [The Wall Street Journal]

And so the fight continues.