MacOS – Can’t get in macOS Utilities

I had a white MacBook with roughly the same specifics (except for a 160 GB HD) and it wouldn't run so slowly.

You can try the following software fixes:

1. Go to System Preferences -> Users and disable unnecessary login items. These items are executed right after login.
2. Go to /Library/StartupItems and disable unnecessary startup items. These items are executed before login, just after boot. This is safe because essential system scripts are not placed there. Don't touch anything in /System/Library, instead.
3. Use Activity Monitor and identify third-party daemons and processes that are running in background. Disable/uninstall unnecessary software.
4. Open Console and look for unusual error messages.
5. Clear ~/Library/Caches and /Library/Caches.
6. Check and — if needed – repair your hard drive. You can do this booting with Recovery HD.
7. Check and repair permissions with Disk Utility.
8. Go to Dashboard and close unnecessary widgets. Some widgets run in background and use system resources.
9. Only if you know what you're doing, you could also check third-party kernel extensions.

Hardware improvements:

1. Buy a SSD. The spinning hard drive is the greatest bottleneck in modern laptops.
2. If you're running short of RAM, consider upgrading to 8 GB if your machine supports it (4 GB should be enough, though).

The thing is that 100% CPU usage refers to a single one of the cores in your CPU being fully loaded (or an equivalent load distributed among all the cores). I assume your MacBook Pro has a dual core CPU with HyperThreading; that's 2 physical cores plus 2 logical cores exposed by HyperThreading. Thus, you could in theory achieve as high as 400% CPU usage (4 cores times 100% each).

As to whether it is sustainable, in principle it should be, if the cooling system is in order and the cooling vents are not clogged with dust or blocked by any objects. Even if it's not and you reach supposedly dangerous temperatures, the CPU should throttle itself to prevent any damage.