diff --git a/benchmark/vm_call_kw_and_kw_splat.yml b/benchmark/vm_call_kw_and_kw_splat.yml
new file mode 100644
index 00000000000000..aa6e549e0c4224
--- /dev/null
+++ b/benchmark/vm_call_kw_and_kw_splat.yml
@@ -0,0 +1,25 @@
+prelude: |
+  h1, h10, h100, h1000 = [1, 10, 100, 1000].map do |n|
+    h = {kw: 1}
+    n.times{|i| h[i.to_s.to_sym] = i}
+    h
+  end
+  eh = {}
+  def kw(kw: nil, **kws) end
+benchmark:
+  1: |
+    kw(**h1)
+  1_mutable: |
+    kw(**eh, **h1)
+  10: |
+    kw(**h10)
+  10_mutable: |
+    kw(**eh, **h10)
+  100: |
+    kw(**h100)
+  100_mutable: |
+    kw(**eh, **h100)
+  1000: |
+    kw(**h1000)
+  1000_mutable: |
+    kw(**eh, **h1000)
diff --git a/test/ruby/test_keyword.rb b/test/ruby/test_keyword.rb
index 956cc9c56da8ec..06bba60ac5c98b 100644
--- a/test/ruby/test_keyword.rb
+++ b/test/ruby/test_keyword.rb
@@ -4002,6 +4002,20 @@ def m(a: [])
     }, bug8964
   end
 
+  def test_large_kwsplat_to_method_taking_kw_and_kwsplat
+    assert_separately(['-'], "#{<<~"begin;"}\n#{<<~'end;'}")
+    begin;
+      n = 100000
+      x = Fiber.new do
+        h = {kw: 2}
+        n.times{|i| h[i.to_s.to_sym] = i}
+        def self.f(kw: 1, **kws) kws.size end
+        f(**h)
+      end.resume
+      assert_equal(n, x)
+    end;
+  end
+
   def test_dynamic_symbol_keyword
     bug10266 = '[ruby-dev:48564] [Bug #10266]'
     assert_separately(['-', bug10266], "#{<<~"begin;"}\n#{<<~'end;'}")
diff --git a/vm_args.c b/vm_args.c
index aa8c2ec2993b72..28c0e339a36a7e 100644
--- a/vm_args.c
+++ b/vm_args.c
@@ -396,6 +396,83 @@ args_setup_kw_parameters(rb_execution_context_t *const ec, const rb_iseq_t *cons
     locals[key_num] = unspecified_bits_value;
 }
 
+static void
+args_setup_kw_parameters_from_kwsplat(rb_execution_context_t *const ec, const rb_iseq_t *const iseq,
+                         VALUE keyword_hash, VALUE *const locals)
+{
+    const ID *acceptable_keywords = ISEQ_BODY(iseq)->param.keyword->table;
+    const int req_key_num = ISEQ_BODY(iseq)->param.keyword->required_num;
+    const int key_num = ISEQ_BODY(iseq)->param.keyword->num;
+    const VALUE * const default_values = ISEQ_BODY(iseq)->param.keyword->default_values;
+    VALUE missing = 0;
+    int i, di;
+    int unspecified_bits = 0;
+    VALUE unspecified_bits_value = Qnil;
+
+    for (i=0; i<req_key_num; i++) {
+        VALUE key = ID2SYM(acceptable_keywords[i]);
+        VALUE deleted_value = rb_hash_delete_entry(keyword_hash, key);
+        if (!UNDEF_P(deleted_value)) {
+            locals[i] = deleted_value;
+        }
+        else {
+            if (!missing) missing = rb_ary_hidden_new(1);
+            rb_ary_push(missing, key);
+        }
+    }
+
+    if (missing) argument_kw_error(ec, iseq, "missing", missing);
+
+    for (di=0; i<key_num; i++, di++) {
+        VALUE key = ID2SYM(acceptable_keywords[i]);
+        VALUE deleted_value = rb_hash_delete_entry(keyword_hash, key);
+        if (!UNDEF_P(deleted_value)) {
+            locals[i] = deleted_value;
+        }
+        else {
+            if (UNDEF_P(default_values[di])) {
+                locals[i] = Qnil;
+
+                if (LIKELY(i < KW_SPECIFIED_BITS_MAX)) {
+                    unspecified_bits |= 0x01 << di;
+                }
+                else {
+                    if (NIL_P(unspecified_bits_value)) {
+                        /* fixnum -> hash */
+                        int j;
+                        unspecified_bits_value = rb_hash_new();
+
+                        for (j=0; j<KW_SPECIFIED_BITS_MAX; j++) {
+                            if (unspecified_bits & (0x01 << j)) {
+                                rb_hash_aset(unspecified_bits_value, INT2FIX(j), Qtrue);
+                            }
+                        }
+                    }
+                    rb_hash_aset(unspecified_bits_value, INT2FIX(di), Qtrue);
+                }
+            }
+            else {
+                locals[i] = default_values[di];
+            }
+        }
+    }
+
+    if (ISEQ_BODY(iseq)->param.flags.has_kwrest) {
+        const int rest_hash_index = key_num + 1;
+        locals[rest_hash_index] = keyword_hash;
+    }
+    else {
+        if (!RHASH_EMPTY_P(keyword_hash)) {
+            argument_kw_error(ec, iseq, "unknown", rb_hash_keys(keyword_hash));
+        }
+    }
+
+    if (NIL_P(unspecified_bits_value)) {
+        unspecified_bits_value = INT2FIX(unspecified_bits);
+    }
+    locals[key_num] = unspecified_bits_value;
+}
+
 static inline void
 args_setup_kw_rest_parameter(VALUE keyword_hash, VALUE *locals, int kw_flag)
 {
@@ -415,22 +492,6 @@ args_setup_block_parameter(const rb_execution_context_t *ec, struct rb_calling_i
     *locals = rb_vm_bh_to_procval(ec, block_handler);
 }
 
-struct fill_values_arg {
-    VALUE *keys;
-    VALUE *vals;
-    int argc;
-};
-
-static int
-fill_keys_values(st_data_t key, st_data_t val, st_data_t ptr)
-{
-    struct fill_values_arg *arg = (struct fill_values_arg *)ptr;
-    int i = arg->argc++;
-    arg->keys[i] = (VALUE)key;
-    arg->vals[i] = (VALUE)val;
-    return ST_CONTINUE;
-}
-
 static inline int
 ignore_keyword_hash_p(VALUE keyword_hash, const rb_iseq_t * const iseq, unsigned int * kw_flag, VALUE * converted_keyword_hash)
 {
@@ -746,15 +807,8 @@ setup_parameters_complex(rb_execution_context_t * const ec, const rb_iseq_t * co
             args_setup_kw_parameters(ec, iseq, args->kw_argv, kw_arg->keyword_len, kw_arg->keywords, klocals);
         }
         else if (!NIL_P(keyword_hash)) {
-            int kw_len = rb_long2int(RHASH_SIZE(keyword_hash));
-            struct fill_values_arg arg;
-            /* copy kw_argv */
-            arg.keys = args->kw_argv = ALLOCA_N(VALUE, kw_len * 2);
-            arg.vals = arg.keys + kw_len;
-            arg.argc = 0;
-            rb_hash_foreach(keyword_hash, fill_keys_values, (VALUE)&arg);
-            VM_ASSERT(arg.argc == kw_len);
-            args_setup_kw_parameters(ec, iseq, arg.vals, kw_len, arg.keys, klocals);
+            keyword_hash = check_kwrestarg(keyword_hash, &kw_flag);
+            args_setup_kw_parameters_from_kwsplat(ec, iseq, keyword_hash, klocals);
         }
         else {
             VM_ASSERT(args_argc(args) == 0);