The phrase "significant slack" refers to having a lot of extra time, resources, or capacity available. It means that there is room for flexibility or more work to be done without being overwhelmed or restricted. Full definition
«Looking ahead, the economy is expected to grow modestly, but with current policy settings the pace will be insufficient to absorb significant slack in the economy, raising the risk of a permanent loss of productive capacity.» (politics.co.uk)
From the past to the future, and to Hell and back... witness the triumphant travels of history's most significant slackers Bill... (bloody-disgusting.com)
For many firms with significant slack assets, they could have resisted this fall in the stock price, but AIG could not. (alephblog.com)