• Bad parameter sniffing.

    http://sqlinthewild.co.za/index.php/2007/11/27/parameter-sniffing/

    https://www.simple-talk.com/sql/t-sql-programming/parameter-sniffing/

    Or maybe stale stats.

    Gail Shaw
    Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
    SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability

    We walk in the dark places no others will enter
    We stand on the bridge and no one may pass