If you've running iOS 11.1 through iOS 11.1.2, whether you have an iPhone X or an iPhone 5S, you'll notice that Apple's Calculator app can't keep up with your calculations. That's a because a bug has seriously slowed it down so that operation buttons won't always trigger when you press them. Luckily, there's something you can do about it.
First, before getting to the fix, let's better explain what the iOS 11 Calculator glitch is, and that's best down with a few examples of some binary operations.
When performing mathematical calculations fast like you normally would, 1 + 2 + 3 would equal 24 instead of 6 because it won't register the second addition sign before you tap 3 and equals. Likewise, if you try to enter 5 x 2 x 3 quickly, you won't get 30, you'll end up with 115 after you hit the equals button.
If you use the iOS 11 Calculator a lot on your iPhone, then this is a huge deal. Luckily, if you don't like tapping realllllly slooooowwww, Apple has issued a patch 32 days after it first appeared. Yes, that's a pretty slow turnaround, but it's better than nothing.
Apple has already fixed this issue in iOS 11.2 which was released to the public on Dec. 2, 2017. So, if you're still running iOS 11.1, 11.1.1, or 11.1.2, make sure you update to the latest version of iOS on your iPhone. Just open up the Settings app, then tap "General," followed by "Software Update," then follow the on-screen instructions to install the latest version.
The rest of the options below were meant to be used before Apple issues a Calculator patch, but you can still use them, of course, if you want.
While you're better off just using the Calculator at a slower pace, you can also choose to use Spotlight Search to perform math calculations. To access it, just swipe down on the screen when you're on a home screen page.
It's not very fast at all since you have to switch between the numbered keyboard and the other characters, but worth mentioning nonetheless. This would only work better for you if you choose to install and use a third-party keyboard such as The Math Keyboard.
If you don't want to join a beta to fix a bug when you could possibly end up with more bugs throughout your system, and you don't want to use Spotlight with a third-party keyboard, then your last option is to just use another calculator app.
There are a ton of them on the iOS App Store, so just search for one that looks good and suits your needs. Obviously, you won't be able to use the Control Center shortcut to open it up, but you can't win at everything (except maybe math).
Some of the best-rated free ones in the iOS App Store include The Calculator, Calculator+ (which also hides photos and videos), and Calculator #. There are also more feature-heavy calculators that cost money, such as Soulver ($2.99) and PCalc ($9.99), but I'd only recommend those if you're taking advanced math classes or deal with mathematical equations like every day.