Task Manager can't replace dedicated tools such as Process Manager, but the application has seen incremental improvements with each major Windows update. In 8 and 10, it even includes a new tab, called "Startup", that lets you see which applications are slowing down the boot process.
Image by Gavin Gear.
To see this information, open Task Manager and select the Startup tab. You'll the see a list of programs that execute when Windows loads, along with a "Startup Impact" rating — Low, Medium or High. It's not that fine-grained and what the values themselves mean specifically is not explained, though it's sufficient to say that you might want to take a look at the ones with High and even Medium scores and decide whether they should be running at all.
In the image above, for example, you could disable EA's gaming client, Origin. It's not something that needs to start with Windows and can be run manually when you want to play a game that requires it.
The good news is this feature isn't going anywhere with Windows 10, so if you just need a rough idea of what programs are hogging your CPU during startup, it's an easy way to gather that information without resorting to a third-party tool.