[Solved]-linq is taking long to order by datediff-LINQ,C#

Rewrite your query to select DTO after ordering:

var longRunningQueries = context.TblMessageQueues
    .GroupBy(x => x.SequencingQueue)
    .Select(g => new {
        DateDiff = DbFunctions.DiffMonths(g.Min(x => x.QueuedTime), g.Max(x => x.QueuedTime)),
        SequencingQueue = g.Key
    }).OrderByDescending(a => a.DateDiff)
    .Select(t => new TblMessageQueueDto {
        DateDiff = t.DateDiff,
        SequencingQueue = t.SequencingQueue
    }).ToList();