Assuming you changed the compatibility setting on your database so that it's now at 120, you're using the new cardinality estimator that comes with SQL Server 2014.

Queries that were marginal in prior versions of SQL Server, and as was pointed out above, there are a lot of tuning opportunities in that query, can react quite badly to the new cardinality estimator (which, is actually much better than the old one). Your best bet, tune the queries. Second best bet, identify the queries that are having problems and use traceflags to put their behavior back to the old cardinality estimator. Last resort, change the compatibility level of your database so that it doesn't use the new cardinality estimator.

Read more about the estimator, get the trace flags.

I'd tune the queries.