Adjust FlatList performance in main feeds (#3134)

* adjust flatlist perf settings

* calculate initial num to render based on screen height

* adjust window size

* don't react to screen height changes

src/lib/hooks/useInitialNumToRender.ts

@@ -0,0 +1,11 @@
+import React from 'react'
+import {Dimensions} from 'react-native'
+
+const MIN_POST_HEIGHT = 100
+
+export function useInitialNumToRender(minItemHeight: number = MIN_POST_HEIGHT) {
+  return React.useMemo(() => {
+    const screenHeight = Dimensions.get('window').height
+    return Math.ceil(screenHeight / minItemHeight) + 1
+  }, [minItemHeight])
+}