在构建一个APP时系统会默认将window的首页指向默认的ViewController，并且在storyboard中勾选Is Initial View Controller。

但是在实际项目中会有navigation或者tabBar很少会只有一个viewController的情况。

那就会出现三种加载情况

1. 直接在storyboard中将指向需要加载的viewController
2. 在- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions修改window.rootViewController。（可以不创建window对象）
3. 去掉Is Initial View Controller选项，创建self.window采用window.rootViewController方式加载。

针对以上三种情况做了一个小Demo实验加载时间。

情况一

直接在storyboard中将指向需要加载的VC，- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions不写任何代码

测试数据

Total pre-main time: 922.79 milliseconds (100.0%)
Total pre-main time: 1.2 seconds (100.0%)
Total pre-main time: 804.45 milliseconds (100.0%)
Total pre-main time: 794.64 milliseconds (100.0%)
Total pre-main time: 293.62 milliseconds (100.0%)
Total pre-main time: 532.53 milliseconds (100.0%)
Total pre-main time: 284.56 milliseconds (100.0%)
Total pre-main time: 523.57 milliseconds (100.0%)
Total pre-main time: 276.15 milliseconds (100.0%)
Total pre-main time: 510.96 milliseconds (100.0%)

最高：1.2
最低：276
平均：614.327
会加载- (instancetype)initWithCoder:(NSCoder *)aDecoder

情况二

在- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions修改window.rootViewController。

self.window.rootViewController = [[ViewController alloc] init];

测试数据

Total pre-main time: 554.46 milliseconds (100.0%)
Total pre-main time: 346.90 milliseconds (100.0%)
Total pre-main time: 670.21 milliseconds (100.0%)
Total pre-main time: 618.56 milliseconds (100.0%)
Total pre-main time: 339.39 milliseconds (100.0%)
Total pre-main time: 668.87 milliseconds (100.0%)
Total pre-main time: 412.73 milliseconds (100.0%)
Total pre-main time: 643.38 milliseconds (100.0%)
Total pre-main time: 360.34 milliseconds (100.0%)
Total pre-main time: 456.20 milliseconds (100.0%)

最高：670
最低：339
平均：507.104
会加载- (instancetype)initWithCoder:(NSCoder *)aDecoder和
- (instancetype)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil

情况三

去掉Is Initial View Controller选项，创建self.window采用window.rootViewController方式加载。

self.window = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];
self.window.rootViewController = [[ViewController alloc] init];
[self.window  makeKeyAndVisible];

测试数据

Total pre-main time: 617.35 milliseconds (100.0%)
Total pre-main time: 293.30 milliseconds (100.0%)
Total pre-main time: 642.83 milliseconds (100.0%)
Total pre-main time: 276.14 milliseconds (100.0%)
Total pre-main time: 567.95 milliseconds (100.0%)
Total pre-main time: 570.26 milliseconds (100.0%)
Total pre-main time: 279.53 milliseconds (100.0%)
Total pre-main time: 476.20 milliseconds (100.0%)
Total pre-main time: 649.68 milliseconds (100.0%)
Total pre-main time: 423.90 milliseconds (100.0%)

最高：649
最低：276
平均：479.714
会加载
- (instancetype)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
报警告
[Application] Failed to instantiate the default view controller for UIMainStoryboardFile 'Main' - perhaps the designated entry point is not set?

总结：
目前只是一个Demo 代码量也不多，而且也考虑的不够严谨，需要结合实际项目才能有更精确的结论(Demo 中启动时间差异较大)。是否能提升APP启动速度需要在项目中尝试。

使用工具：
10.14.3 (18D109)
MacBook Pro (13-inch, 2017, Two Thunderbolt 3 ports)
2.3 GHz Intel Core i5
8 GB 2133 MHz LPDDR3
Intel Iris Plus Graphics 640 1536 MB

Xcode Version 10.2 (10E125)
Simulator iPhoine X iOS 12.2
Debug 环境