[nicolas-grekas]

$vars + $context + $this->env->getGlobals() copies the whole context twice. Adding the globals in place saves one copy per with tag, 2.2k instructions with a context of 40 variables.

[GromNaN]

Nice improvement, the two statements avoid one full copy of the context.

An optional idea, not blocking: part of the work could be decided at compile time.

1. With only, the context is known to be empty, so the generated code could be reduced to a single statement and skip the $context = []; assignment:

   $context = $vars + $this->env->getGlobals();

   The precedence stays the same (with variables, then globals).

2. When the variables node is an empty ArrayExpression ({% with {} %}), WithNode::compile() could skip the is_iterable() check, toArray() and the context merge entirely, and only add the globals when only is set.

The second case is rare in real templates, and a variable that is empty at runtime cannot be detected by the compiler, so I would not expect a measurable gain there. The first case is cheap to implement and makes the generated code shorter.