I find that comment hard to give credence to - when designing for performance we'll squeeze every last ounce out of the design we can - I can't imagine anyone's going to give up landing distance for something like that.
Are you sure he wasn't referring to in-air deployment of speedbrakes - where a slower deployment might reduce the 'g' bump on deployment, and might be a comfort, or even handing, issue (at least one of our aircraft has low rate spoiler deployment in-air for much those reasons) - rather than in GLD mode?