Comparando implementações que obtem os dias úteis de um ano de maneira iterativa, com streams/flatMap e com streams/collectToMap. Uma medição simples de 100 iterações em um i5 com 4 processadores teve o seguinte resultado: iterativo ~ 47 ms, com flatMap ~ 77 ms, com collect toMap ~ 51 ms.

DiasUteisComCollectToMap.java

import java.time.DayOfWeek;
import java.time.Month;
import java.time.YearMonth;
import java.util.function.Function;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class DiasUteisComCollectToMap {
	public static void comCollectToMap(int ano) {
		Stream.of(Month.values())
			.map(month -> YearMonth.of(ano, month))
			.collect(Collectors.toMap(Function.identity(),
				ym -> Stream.iterate(ym.atDay(1),  date -> date.plusDays(1)).limit(ym.lengthOfMonth())
				.filter(date -> !date.getDayOfWeek().equals(DayOfWeek.SATURDAY) && !date.getDayOfWeek().equals(DayOfWeek.SUNDAY))
			    .collect(Collectors.toList())));
	}
}

DiasUteisComFlatMap.java

import java.time.DayOfWeek;
import java.time.Month;
import java.time.YearMonth;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class DiasUteisComFlatMap {

	public static void comFlatMap(int ano) {
		Stream.of(Month.values())
			.map(month -> YearMonth.of(ano, month))
			.flatMap(ym -> Stream.iterate(ym.atDay(1),  date -> date.plusDays(1)).limit(ym.lengthOfMonth()))
			.filter(date -> !date.getDayOfWeek().equals(DayOfWeek.SATURDAY) && !date.getDayOfWeek().equals(DayOfWeek.SUNDAY))
			.collect(Collectors.groupingBy(date -> YearMonth.of(date.getYear(), date.getMonth())));
	}
}

DiasUteisIterativo.java

import java.time.DayOfWeek;
import java.time.LocalDate;
import java.time.Month;
import java.time.YearMonth;
import java.util.ArrayList;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;

public class DiasUteisIterativo {

	public static void iterativo(int ano) {
		Map<YearMonth, List<LocalDate>> iterativo = new LinkedHashMap<>();
		for(Month mes : Month.values()) {
			YearMonth anoMes = YearMonth.of(ano, mes);
			List<LocalDate> diasUteisDoAno = new ArrayList<>();
			for(int dia=1; dia <= anoMes.lengthOfMonth(); dia++){
				LocalDate date = anoMes.atDay(dia);
				if(!date.getDayOfWeek().equals(DayOfWeek.SATURDAY) && !date.getDayOfWeek().equals(DayOfWeek.SUNDAY)){
					diasUteisDoAno.add(date);
				}
			}
			iterativo.put(anoMes, diasUteisDoAno);
		}
	}
}

MedidorDeTempo.java

import java.time.Duration;
import java.time.Instant;

public class MedidorDeTempo {
	public static long mede(Runnable algoritmo){
		Instant inicio = Instant.now();
		for(int i = 0; i < 100; i++){
			algoritmo.run();
		}
		Instant fim = Instant.now();
		return Duration.between(inicio, fim).toMillis();
	}
}

Principal.java

import java.util.stream.Stream;
import static DiasUteisIterativo.*;
import static DiasUteisComFlatMap.*;
import static DiasUteisComCollectToMap.*;

public class Principal {

	public static void main(String[] args) {
	    
		Stream.<Runnable>of(
				() -> iterativo(2014), // ~ 47 ms
				() -> comFlatMap(2014), // ~ 77 ms
				() -> comCollectToMap(2014)) // ~ 51 ms
		    .map(MedidorDeTempo::mede)
		    .forEach(System.out::println);
		    
	    //by @paulo_caelum
	}
}