"slightly outperformed" is a grammatically correct and usable phrase in written English.
You can use it when you want to express that something was slightly better than something else. For example: "The new product slightly outperformed the old one in terms of sales."
Elgie: B.C.'s GDP has slightly outperformed the rest of Canada during that time period.
(Democrats slightly outperformed the polling average in Wisconsin in both 2004 and 2008).
Among IPOs completed from 1980 through 2014, issuers with annual revenue over $500m slightly outperformed the market.
In both 2012 and 2013 boys slightly outperformed girls in the proportion of A* grades awarded, by slim margins.
For the first time since the A* was introduced in 2010, boys slightly outperformed girls at the grade.
The brainstorming groups slightly outperformed the groups given no instructions, but teams given the debate condition were the most creative by far.
Bucking expectations of the revised curriculum, girls slightly outperformed boys, with 30% achieving an A* or A grade compared to 29.5% of of boys.
Although its overall well-being score at 71.6 was lower than Britain's 81.1, Poland also slightly outperformed the UK on the health dimension of the Seda score.
Non-dividend paying stocks slightly outperformed dividend-paying stocks, and many corporations that did pay higher dividends scaled back stock repurchases by a similar amount.
The data collected from the reading comprehension text was analyzed statistically and the results showed that the experimental group students slightly outperformed the control group students.
The remark was widely condemned by teachers and former pupils of girls' schools across England, and a subsequent analysis of pupils' results in 2015 found that all-girl secondary schools slightly outperformed those for boys.
