Update docs and tests.

doc/src/start/builds/performance.md

@@ -3,11 +3,12 @@ Performance Build
 
 {{#include ../../links.md}}
 
+Some features are for performance.  For example, using [`only_i32`] or [`only_i64`] disables all other integer types (such as `u16`).
+
+
 Use Only One Integer Type
 ------------------------
 
-Some features are for performance.  For example, using [`only_i32`] or [`only_i64`] disables all other integer types (such as `u16`).
-
 If only a single integer type is needed in scripts - most of the time this is the case - it is best to avoid registering
 lots of functions related to other integer types that will never be used.  As a result, [`Engine`] creation will be faster
 because fewer functions need to be loaded.
@@ -22,8 +23,8 @@ On 64-bit targets this may not gain much, but on some 32-bit targets this improv
 requiring more CPU cycles to complete.
 
 
-Minimize Size of [`Dynamic`]
----------------------------
+Minimize Size of `Dynamic`
+-------------------------
 
 Turning on [`no_float`], and [`only_i32`] makes the key [`Dynamic`] data type only 8 bytes small on 32-bit targets
 while normally it can be up to 16 bytes (e.g. on x86/x64 CPU's) in order to hold an `i64` or `f64`.
@@ -42,3 +43,15 @@ It is cheap to clone, but expensive to modify (a new copy of the string must be
 
 Therefore, functions taking `String` parameters should use `ImmutableString` or `&str` (which maps to `ImmutableString`)
 for the best performance with Rhai.
+
+
+Disable Closures
+----------------
+
+Support for [closures], including capturing shared values, adds material overhead to script evaluation.
+
+This is because every data access must be checked whether it is a shared value, and if so, take a read
+or write lock before reading it.
+
+Use [`no_closure`] to disable closure and capturing support and optimize the hot path
+because there is no need to take locks for shared data.