IEnumerableSlowdown-Linqpad

    IEnumerable<string> fullList = GetRandomStrings(10000);
    IEnumerable<string> comparisonList = GetRandomStrings(10000);

    var inefficientTime = ProcessList_Inefficient(fullList,comparisonList);
    var efficientTime = ProcessList_Efficient(fullList,comparisonList);

    var msg1 = "Inefficient took "+inefficientTime.ToString() + " milliseconds";
    var msg2 = "Efficient took "+efficientTime.ToString() + " milliseconds";

    msg1.Dump();
    msg2.Dump();
}

IEnumerable<string> GetRandomStrings(int count)
{
    for(int i = 0; i < count;i++){
        yield return Path.GetRandomFileName();
    }
}

long ProcessList_Inefficient(IEnumerable<string> firstList, IEnumerable<string> comparisonList){
    Stopwatch sw = Stopwatch.StartNew();
    firstList.ToList().RemoveAll(x=>{
        if(comparisonList.Contains(x)){
            return true;
        }
        return false;
    });
    return sw.ElapsedMilliseconds;
}

long ProcessList_Efficient(IEnumerable<string> firstList, IEnumerable<string> comparisonList){
    Stopwatch sw = Stopwatch.StartNew();
    var comparisonListAsList = comparisonList.ToList();

    firstList.ToList().RemoveAll(x=>{
        if(comparisonListAsList.Contains(x)){
            return true;
        }
        return false;
    });
    return sw.ElapsedMilliseconds;
}

class Closing{