"slightly underperforming" is correct and usable in written English.
You can use it to describe something that is not meeting the expected standards, like a student who is "slightly underperforming in their math class".
Mr. Obama may also be just slightly underperforming the fundamentals in the election.
Its shares have added 7p to 536.5p, albeit slightly underperforming what is at the moment a booming stock market.
Goldman's Sustain fund is up 4.57 percent year to date, slightly underperforming its benchmark, the MSCI All Country World Index, according to Trustnet Offshore, a Web site that tracks offshore fund returns.
In 2012 the firm returned about 13 percent net of fees, which, while slightly underperforming the Standard & Poor's 500-stock index, was superior to the results of the average hedge fund.
Another strike against Comcast is its stock performance, which has plunged more than 12% this year, while Disney has gained about 3%, slightly underperforming the broader S&P 500 Index, which has risen about 5%.
Hon Hai Precision fell 0.48percentt on Monday, slightly underperforming a 0.14percentt fall in the main Taiwan stock index.
Since 2013, Pfizer's stock has slightly underperformed the Standard & Poor's 500-stock index.
There were a number of states, like Oklahoma, Tennessee and Kentucky, in which Mr. Obama slightly underperformed in the final polls.
Nestlé, a Swiss food company, slightly underperforms its big competitors in good times but outperforms them in bad.
"In fact, the whole of east Kent has slightly underperformed against the rest of the south-east since HS1 opened," said Richard Wellings of the thinktank the Institute for Economic Affairs.
Basically, for a variety of possible reasons, players tend to slightly underperform during the initial portion of a long-term contract.
