If you're in compat mode 100, then it's not the cardinality estimator. That's usually what causes performance regressions in 2014. The traceflag Phil suggested won't help if it's not the cardinality estimator.

Can you pull the execution plan? Also, what wait types does the query incur?

Also, going back to 2008 won't be an easy process. You'll need a pre-upgrade backup and some way to migrate all data changes made since the upgrade.