Update Best Practices and FAQ.md

Guides/Best Practices and FAQ.md

@@ -31,19 +31,19 @@ func didUpdate(to object: Any) {
 
 ## Frequently asked questions
 
-**How do you implement separators between cells?**
+#### How do you implement separators between cells?
 
 See discussion in [#329](https://github.com/Instagram/IGListKit/issues/329)
 
-**How do I fix the error `Could not build Objective-C module 'IGListKit'`?**
+#### How do I fix the error `Could not build Objective-C module 'IGListKit'`?
 
 See discussion in [#316](https://github.com/Instagram/IGListKit/issues/316)
 
-**The documentation and examples have `<X>` feature or changes, but I don't have it in my version. Why?**
+#### The documentation and examples have `<X>` feature or changes, but I don't have it in my version. Why?
 
 This feature is on the `master` branch only and hasn't been officially tagged and [released](https://github.com/Instagram/IGListKit/releases). If you need to, you can [install from the `master` branch](https://instagram.github.io/IGListKit/installation.html).
 
-**Does `IGListKit` work with...?**
+#### Does `IGListKit` work with...?
 
 - Core Data ([Working with Core Data](https://instagram.github.io/IGListKit/working-with-core-data.html) Guide)
 - AsyncDisplayKit ([AsyncDisplayKit/#2942](https://github.com/facebook/AsyncDisplayKit/pull/2942))
@@ -54,14 +54,18 @@ This feature is on the `master` branch only and hasn't been officially tagged an
 
 Yes.
 
-**Does `IGListKit` work with `UITableView`?**
+#### Does `IGListKit` work with `UITableView`?
 
 No, but you can install the [diffing subspec via CocoaPods](https://instagram.github.io/IGListKit/installation.html).
 
-**What's the purpose of `IGListCollectionView`?**
+#### What's the purpose of `IGListCollectionView`?
 
 We use this subclass to gain compile-time safety to prevent disallowed methods from being called on `UICollectionView`, because `IGListKit` handles model and view updates. See discussion at [#409](https://github.com/Instagram/IGListKit/issues/409).
 
-**How can I manage cell selection and deselection?**
+#### How can I manage cell selection and deselection?
 
 See discussion at [#184](https://github.com/Instagram/IGListKit/issues/184).
+
+#### I have a *huge* data set and [`-peformUpdatesAnimated: completion:`](https://instagram.github.io/IGListKit/Classes/IGListAdapter.html#/c:objc(cs)IGListAdapter(im)performUpdatesAnimated:completion:) is *super* slow. What do I do?
+
+If you have multiple thousands of items and you cannot batch them in, you'll see performance issues with `-peformUpdatesAnimated: completion:`. The real bottle neck behind the scenes here is `UICollectionView` attempting to insert so many cells at once. Instead, call [`-reloadDataWithCompletion:`](https://instagram.github.io/IGListKit/Classes/IGListAdapter.html#/c:objc(cs)IGListAdapter(im)reloadDataWithCompletion:) when you first load data, which will be super fast. Behind the scenes, this method *does not* do any diffing and simply calls `-reloadData` on `UICollectionView`. For subsequent updates, you can then use `-peformUpdatesAnimated: completion:`.