This db query takes way too long

Question

I have a query for a details-view of a meeting, which has a lot of included related data:

Meeting meeting = await db.Meetings
    .Include(a => a.Agenda)
        .ThenInclude(s => s.Speakers)
            .ThenInclude(u => u.User)
    .Include(s => s.Summonings)
        .ThenInclude(u => u.User)
    .Where(p => p.Id == parsedMeetingId)
    .FirstOrDefaultAsync();

I have set up a Stopwatch, and these are the results for the last six runs (SS:MS):

25:23
25:04
24:36
00:03 // What happened here, I really don't know!
25:25
25:06

There is not much data in the db; Only one Meeting with 4 Summonings, an Agenda of 11 AgendaItems, and only 1 Speaker in 1 AgendaItem.

My dev PC is a rather modest AMD laptop with only 8 GB RAM, running Windows 10. At the time of testing, I was only running Visual Studio and Chrome.

(How) can the query or model design be improved?

Where can I look for other bootle necks?

The models:

public class Meeting
{
    public Guid Id { get; set; }
    public Guid OwnerId { get; set; }
    public Guid ModeratorId { get; set; }
    public Guid ReportWriterId { get; set; }
    public string Title { get; set; }
    public List<AgendaItem> Agenda { get; set; }
    public List<MeetingSummoning> Summonings { get; set; }
    public ApplicationUser Owner { get; set; }
    public ApplicationUser Moderator { get; set; }
    public ApplicationUser ReportWriter { get; set; }
    // Some more properties ...
}

public class AgendaItem
{
    public Guid Id { get; set; }
    public string Title { get; set; }
    public Guid MeetingId { get; set; }
    public Meeting Meeting { get; set; }
    public List<Speaker> Speakers { get; set; }
    // Some more properties ...
}

public class MeetingSummoning
{
    public Guid Id { get; set; }
    public Guid MeetingId { get; set; }
    public Guid UserId { get; set; }
    public Meeting Meeting { get; set; }
    public ApplicationUser User { get; set; }
}

public class Speaker
{
    public Guid Id { get; set; }
    public DateTime Requested { get; set; }
    public DateTime? Started { get; set; }
    public DateTime? Finished { get; set; }
    public bool IsCurrent { get; set; }
    public Guid? ReplyToSpeakerId { get; set; }
    public Speaker ReplyToSpeaker { get; set; }
    public Guid AgendaItemId { get; set; }
    public AgendaItem AgendaItem { get; set; }
    public Guid UserId { get; set; }
    public ApplicationUser User { get; set; }
}

public class ApplicationUser : IdentityUser<Guid>
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public List<MeetingSummoning> MeetingSummonings { get; set; }
    public List<Meeting> OwnedMeetings { get; set; }
    public List<Meeting> ModeratedMeetings { get; set; }
    public List<Meeting> ReportedMeetings { get; set; }
    // Lots more properties ...
}