Entity Framework Core 3.1.3 very slow first query used within AWS Lambda Function(s) and AWS API Gateway serverless API

I encountered this exact issue (hence bringing me here).My lambda was only allocated 128MB and was taking 17 seconds to build up EF.On upping the memory to 2048MB, the same process was reduced to 1 second.The lambda actually only required 168 MB but of course, that's more than the original 128MB.

please try this...

var dupeEmailCustomers = ( from c in loyalty.ContactInformation                        join cu in loyalty.Customer on c.CustomerInternalId equals cu.CustomerInternalId                        where cu.Status == Customer.CustomerStates.Active                                && c.ContactType == "EMAIL"                        select c                        ).AsNoTracking()                    .Union(                        from c in loyalty.ContactInformation                        join cu in loyalty.Customer on c.CustomerInternalId equals cu.CustomerInternalId                        where c.ContactType == "EMAIL"                               && cu.Status != Customer.CustomerStates.Active                                && testArray.Contains(cu.StatusReason)                        select c                        ).AsNoTracking();

test the timing on these seperately

    from c in loyalty.ContactInformation    join cu in loyalty.Customer on c.CustomerInternalId equals cu.CustomerInternalId    where cu.Status == Customer.CustomerStates.Active            && c.ContactType == "EMAIL"    select c

and

    from c in loyalty.ContactInformation    join cu in loyalty.Customer on c.CustomerInternalId equals cu.CustomerInternalId    where c.ContactType == "EMAIL"           && cu.Status != Customer.CustomerStates.Active            && testArray.Contains(cu.StatusReason)    select c

you could be missing index.... try add index on ContactType and Status and check out if you need one on StatusReason