Performance on iOS is smooth because the software is optimized for the hardware and that's because Apple makes both and they can tailor a software for each device they have because they don't have much devices, while on Android, Google releases the software to everyone and there is a wide variety of hardware manufactures and Android should work on them all also manufactures fill the OS with their bloatware and that slows down the phone, but you'll find optimized Android phones like Google Pixel and OnePlus phones and generally any phone that uses the stock Android software.

---

For multitasking, i dont know what do you exactly mean, do you mean you could use an app with another app on the same screen? if yes; then yeah you can do that on both OSs, but if you mean switching from an app to another while the previous one is running; check this answer [ https://www.quora.com/What-are-the-differences-between-Android-and-iOS-in-multitasking-Conventional-wisdom-says-Android-has-real-multitasking-and-iOS-does-not-What-does-that-mean-What-are-the-pros-and-cons-of-Android-vs-iOS-multitasking ].

---

And for apps diversity, you'll almost find everything you need on both OSs, but some apps are better on iOS than Android in terms of taking pics and videos despite that there are Android phones have better cameras than iPhones and that's because of this [ https://www.google.jo/amp/s/www.androidcentral.com/why-video-quality-instagram-and-snapchat-so-much-worse-android-ios%3famp?espv=1 ].
And also the developers are lazy like askfm devs that didn't bother themselves allowing Android users to upload videos from their phones.
For me, I don't take pics with instagram or snapchat so I don't really care of that problem, so I got an Android phone and saved much money than getting an iPhone that I'd use it the same way I'm using my Android phone right now, my phone isn't ultra smooth like Pixel or OnePlus phones but it doesn't lag even when there's only 500mb free of memory and it's great for me