Does the conversion from DbSet to IEnumerable query?

I have two methods in my log repository.

public IEnumerable<Log> GetAll()
{
   var db = new CasLogEntities();
   return db.Logs;
}           


public DbSet<Log> GetAllSet()
{
   var db = new CasLogEntities();
   return db.Logs;
}           

The only difference is that one returns an IEnumerable from the log, and the other returns the DbSet of the log.

In My Asset Controller, I have the following code

var allLogs = _logRepo.GetAllSet();
var Logs = (from log in allLogs
              group log by log.DeviceId
              into l
              select new {DeviceId = l.Key, TimeStamp = l.Max(s => s.TimeStamp)}).ToList();

Now the problem is that I get a huge difference in performance in the application group, depending on which of the repo methods I call.

• getAllSet, which returns a DbSet, lightning fast,
• GetAll returns IEnumerable more slowly.

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