HONG KONG (Reuters Breakingviews) - Companies around the world just got a stark reminder about how stretched executives can get. Australian billionaire James Packer has resigned from a whopping 22 directorships after quitting the board of casino operator Crown Resorts for mental health reasons. Campaigns against such so-called “overboarding” are on the rise, especially as evidence suggests returns can suffer.

Packer’s situation is a little different than the ones that most bother big fund managers. Despite having previously served as a director at Qantas and elsewhere, his more recent board seats were at private entities, many of them interrelated. Even so, such roles still entail responsibilities and commitments that tax schedules and mindshare.

At public companies, the distraction is even worse. It takes an average of 275 hours a year to prepare for and attend meetings on just one board, the National Association of Corporate Directors found a few years ago.

A backlash against multiple directorships is gathering steam. Proxy adviser Institutional Shareholder Services now recommends investors vote against or withhold support from directors on five or more boards. It is tracking over 50 individuals who sit on at least 10 of them. On its naughty list are Liberty Media Chief Executive Greg Maffei, Mawson Resources Chief Financial Officer Nick DeMare and AsiaLink Capital Chairman Fan Renda. The likes of BlackRock also have recently elevated the issue, challenging directors at Pfizer, AvalonBay and dozens of other companies for overextending themselves.

There are good reasons for concern. A recent study by consultancy Equilar found that companies whose CEOs hold no more than one external directorship generated a one-year median shareholder return of more than 15 percent, assuming dividends were reinvested, on par with the broader market. Bosses serving on multiple boards disappointed by comparison, delivering slightly over 8 percent.

A growing number of the S&P 500 have been wisely cracking down on the practice of overboarding, according to executive search firm Spencer Stuart. This latest example Down Under should hammer home the message.