docs(ivy): add a note about exporting top level variables

packages/core/src/render3/PERF_NOTES.md

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+
+## General Notes
+
+Each Array costs 70 bytes and is composed of `Array` and `(array)` object
+  * `Array` javascript visible object: 32 bytes
+  * `(array)` VM object where the array is actually stored in: 38 bytes
+
+Each Object cost is 24 bytes plus 8 bytes per property.
+
+For small arrays, it is more efficient to store the data as a linked list
+of items rather than small arrays. However, the array access is faster as
+shown here: https://jsperf.com/small-arrays-vs-linked-objects
+
+## Monomorphic vs Megamorphic code
+
+Great read: [What's up with monomorphism?](http://mrale.ph/blog/2015/01/11/whats-up-with-monomorphism.html)
+
+1) Monomorphic prop access is 100 times faster then megamorphic.
+2) Monomorphic call is 4 times faster the megamorphic call.
+
+ See benchmark [here](https://jsperf.com/mono-vs-megamorphic-property-access).
+
+ ## Exporting top level variables
+
+ Exporting top level variables should be avoided where possible where performance
+ and code size matters:
+
+ ```
+ // Typescript
+export let exported = 0;
+let notExported = 0;
+
+notExported = exported;
+
+// Would be compiled to
+exports.exported = 0;
+var notExported = 0;
+
+notExported = exports.exported;
+```
+
+Most minifiers do not rename properties (closure is an exception here).
+
+What could be done instead is:
+
+```
+let exported = 0;
+
+export function getExported() { return exported; }
+export function setExported(v) { exported = v; }
+```
+
+Also writing to a property of `exports` might change its hidden class resulting in megamorphic access.