Linq query with several left join generate few query

I try write query by linq on asp core 2 with Entity framework Core and all ok, query return right result, but very slowly.

var Query1 =
        from demand in _prabacontext.Demand
        join demandspec in _prabacontext.DemandSpec on demand.Id equals demandspec.DemandId into demandspecGroup
        join manager in _prabacontext.Manager on demand.ManagerId equals manager.Id into managerGroup
        from m in managerGroup.DefaultIfEmpty()
        join contractor in _prabacontext.Contractor on demand.ContractorId equals contractor.Id into contractorGroup
        from c in contractorGroup.DefaultIfEmpty()
        join company in _prabacontext.Company on c.CompanyId equals company.Id into companyGroup
        from cm in companyGroup.DefaultIfEmpty()
    where demand.RegDate >= Convert.ToDateTime("2018-01-01T00:00:00")
        select new
        {
            demand.Id,
            demand.Id1,
            RegDate = demand.RegDate.ToString("yyyy-MM-dd"),
            demand.IsLgot,
            demand.ContractorId,
            demand.DocNumber,
            demand.DocStatus,
            demand.ManagerId,
            demand.Remark,
            StrSum = demandspecGroup.Sum(x => (decimal?)(x.Price * x.Quantity)) ?? 0,
            ContractorName = cm.Name,
            ManagerName = m.Name1//+' '+m.Name2+' '+m.Name3
        };

In debugger log this linq query translate into this SQL statement:

SELECT 
    "demand"."_ID" AS "Id0", 
    "demand"."Contractor_ID" AS "ContractorId", 
    "demand"."DocNumber", 
    "demand"."DocStatus", 
    "demand"."ID" AS "Id1", 
    "demand"."Insert_Date", 
    "demand"."Insert_User", 
    "demand"."IsLgot", 
    "demand"."IsTender", 
    "demand"."last_updated", 
    "demand"."Manager_ID" AS "ManagerId", 
    "demand"."RegDate", 
    "demand"."Remark" AS "Remark0", 
    "demandspec"."_ID", 
    "demandspec"."Commerc_ID", 
    "demandspec"."DeliveryDate", 
    "demandspec"."_Demand_ID", 
    "demandspec"."Demand_ID", 
    "demandspec"."FinType_ID", 
    "demandspec"."Goods2_ID", 
    "demandspec"."Goods_ID", 
    "demandspec"."GoodsUnion_ID", 
    "demandspec"."ID", 
    "demandspec"."last_updated", 
    "demandspec"."Price", 
    "demandspec"."Quantity", 
    "demandspec"."Remainder", 
    "demandspec"."Remark", 
    "demandspec"."Reserv"
FROM
    "Demand" AS "demand"
LEFT JOIN 
    "Demand_Spec" AS "demandspec" ON "demand"."_ID" = "demandspec"."_Demand_ID"
ORDER BY 
    "Id0"

SELECT "manager"."id", "manager"."name1"
FROM "manager" AS "manager"

SELECT "contractor"."id", "contractor"."company_id"
FROM "maxim"."contractor" AS "contractor"

SELECT "company"."id", "company"."name"
FROM "maxim"."company" AS "company"`

It returns data from query and programmatically join output data. But this table is very large. I know I can write a SQL query directly, but I want use linq.

How I can write a Linq query what translate into a SQL query with a few left joins?

Added:

In debug log I find such strings:

The LINQ expression 'DefaultIfEmpty()' could not be translated and will be evaluated locally

For every tables in query. How I can rewrite my query without this construction?