WPF: Loading csv file too slow

I have a WPF app that tries to load large data from a csv file (I need for the data to be in memory as client requested). I already have a library that does this process, with the help of CsvHelper

When I run a console application it take less than a second to load, but when I use the same library with WPF, it's very slow takes about 18 seconds.

Is this normal? and if yes is there some sort of fix or should I just give up and accept that it's the way WPF is?

Note: The Core library that does the load is in .NET core, I don't know if it's the problem.

EDIT:

Here is my code for importing the data.

    public void ImportBookingData(Action<BookingRecord> processData, string path)
    {
        using (TextReader reader = new StreamReader(path))
        {
            CsvReader csv = new CsvReader(reader);
            csv.Configuration.TypeConverterCache.AddConverter<DateTime>(new ShortDateTimeCsvConverter());
            csv.Configuration.TypeConverterCache.AddConverter<TimeSpan>(new ShortTimeSpanConverter());

            csv.Configuration.Delimiter = ";";

            while (csv.Read())
            {
                processData.Invoke(csv.GetRecord<BookingRecord>());
            }
        }
    }

To process the incoming records I use this code

        importService.ImportBookingData(record =>
        {

            //Add record to indexed records
            ConstructIndexedRecords(record);

            //The start and end date will take the first record date in the initial stage
            if (firstRecord)
            {
                StartDate = record.FlightDate;
                EndDate = record.FlightDate;
            }

            //Search for time period of the input data records
            CheckForLimitDates(record);

            firstRecord = false;

        }, path);

So I tied to place a stopwatch in various places to pinpoint where the problem is, I tested the speed of csv.Read() and also the speed of reading all the records, and the results were the same. In the processing of the records, I put a stop watch to test the speed of processing one record, and the results were also the same. Finely when I test the entire time of the processing I get less then a second for the Console app, and 10 seconds for WPF. So what do you think? Does it has to do anything with using delegates?