diff --git a/bibtexparser/entrypoint.py b/bibtexparser/entrypoint.py
index 46df5fd..68d9e10 100644
--- a/bibtexparser/entrypoint.py
+++ b/bibtexparser/entrypoint.py
@@ -1,4 +1,5 @@
 import codecs
+import logging
 import warnings
 from collections.abc import Iterable
 from copy import deepcopy
@@ -16,9 +17,16 @@
 from .writer import BibtexFormat
 from .writer import write
 
+logger = logging.getLogger(__name__)
+
 #: Marks a seeded copy of a pre-existing `@string`, dropped before merging back.
 _PREEXISTING_STRING_KEY = "bibtexparser_preexisting_string"
 
+#: Number of blocks from which on `write_string`/`write_file` warn if the unparse
+#: stack deep-copies blocks. Copying costs roughly 30-60 µs per entry, i.e. it
+#: starts to dominate the write time at this size.
+LARGE_LIBRARY_WARNING_THRESHOLD = 10_000
+
 
 def _build_parse_stack(
     parse_stack: Iterable[Middleware] | None,
@@ -80,6 +88,29 @@ def _build_unparse_stack(
     return list(prepend_middleware) + list(unparse_stack)
 
 
+def _warn_if_large_library_is_copied(library: Library, unparse_stack: list[Middleware]) -> None:
+    """Warn if writing ``library`` will deep-copy its blocks and that is likely slow.
+
+    Middlewares with ``allow_inplace_modification=False`` (the default unparse stack
+    is built that way) deep-copy every block they transform, which dominates the
+    write time of large libraries.
+    """
+    n_blocks = len(library.blocks)
+    if n_blocks < LARGE_LIBRARY_WARNING_THRESHOLD:
+        return
+    if all(middleware.allow_inplace_modification for middleware in unparse_stack):
+        return
+    logger.warning(
+        f"Writing a library with {n_blocks} blocks: the unparse stack deep-copies blocks "
+        "(it contains middlewares with allow_inplace_modification=False), "
+        "which is slow for large libraries. "
+        "If you do not need the library after writing, pass an unparse stack whose "
+        "middlewares all allow in-place modification, e.g. "
+        "`unparse_stack=bibtexparser.middlewares.default_unparse_stack("
+        "allow_inplace_modification=True)`."
+    )
+
+
 def _handle_deprecated_write_params(
     unparse_stack: Iterable[Middleware] | None,
     prepend_middleware: Iterable[Middleware] | None,
@@ -287,6 +318,9 @@ def write_file(
                         Only applicable if `unparse_stack` is None.
     :param bibtex_format: Customized BibTeX format to use (optional).
     :param encoding: Encoding of the .bib file. Default encoding is ``"UTF-8"``.
+    Writing a library with at least ``LARGE_LIBRARY_WARNING_THRESHOLD`` blocks logs a warning
+    if the unparse stack deep-copies blocks (middlewares with ``allow_inplace_modification=False``),
+    as that is slow; pass an all-in-place stack to avoid it.
 
     .. deprecated:: (next version)
         Parameters 'parse_stack' and 'append_middleware' are deprecated, will be deleted soon.
@@ -324,6 +358,9 @@ def write_string(
     :param prepend_middleware: List of middleware to prepend to the default stack.
                         Only applicable if `unparse_stack` is None.
     :param bibtex_format: Customized BibTeX format to use (optional).
+    Writing a library with at least ``LARGE_LIBRARY_WARNING_THRESHOLD`` blocks logs a warning
+    if the unparse stack deep-copies blocks (middlewares with ``allow_inplace_modification=False``),
+    as that is slow; pass an all-in-place stack to avoid it.
 
     .. deprecated:: (next version)
         Parameters 'parse_stack' and 'append_middleware' are deprecated.
@@ -333,8 +370,11 @@ def write_string(
         unparse_stack, prepend_middleware, kwargs, "write_string"
     )
 
+    stack = _build_unparse_stack(unparse_stack, prepend_middleware)
+    _warn_if_large_library_is_copied(library, stack)
+
     middleware: Middleware
-    for middleware in _build_unparse_stack(unparse_stack, prepend_middleware):
+    for middleware in stack:
         library = middleware.transform(library=library)
 
     return write(library, bibtex_format=bibtex_format)
diff --git a/tests/test_entrypoint.py b/tests/test_entrypoint.py
index b4656a7..c8a46ab 100644
--- a/tests/test_entrypoint.py
+++ b/tests/test_entrypoint.py
@@ -1,5 +1,6 @@
 """Testing the parse_file and write_file functions."""
 
+import logging
 import os
 import tempfile
 import warnings
@@ -10,7 +11,9 @@
 from bibtexparser import parse_string
 from bibtexparser import write_file
 from bibtexparser import write_string
+from bibtexparser.entrypoint import LARGE_LIBRARY_WARNING_THRESHOLD
 from bibtexparser.library import Library
+from bibtexparser.middlewares import default_unparse_stack
 from bibtexparser.model import DuplicateBlockKeyBlock
 from bibtexparser.model import Entry
 from bibtexparser.model import Field
@@ -401,3 +404,70 @@ def test_parse_string_into_existing_library_keeps_block_order():
         "Entry",
     ]
     assert [entry.key for entry in library.entries] == ["first", "second"]
+
+
+def _library_with_n_entries(n: int) -> Library:
+    return Library(
+        [
+            Entry(entry_type="article", key=f"key{i}", fields=[Field(key="title", value="T")])
+            for i in range(n)
+        ]
+    )
+
+
+def test_large_library_warning_threshold_is_reasonable():
+    assert 1_000 <= LARGE_LIBRARY_WARNING_THRESHOLD <= 100_000
+
+
+def test_write_string_warns_for_large_library_with_copying_stack(monkeypatch, caplog):
+    """The default unparse stack deep-copies blocks, which is slow for large libraries."""
+    monkeypatch.setattr("bibtexparser.entrypoint.LARGE_LIBRARY_WARNING_THRESHOLD", 5)
+    library = _library_with_n_entries(5)
+
+    with caplog.at_level(logging.WARNING, logger="bibtexparser.entrypoint"):
+        write_string(library)
+
+    warnings_ = [r for r in caplog.records if r.levelno == logging.WARNING]
+    assert len(warnings_) == 1
+    assert "Writing a library with 5 blocks" in warnings_[0].message
+    assert "allow_inplace_modification=True" in warnings_[0].message
+
+
+def test_write_file_warns_for_large_library_with_copying_stack(monkeypatch, caplog):
+    monkeypatch.setattr("bibtexparser.entrypoint.LARGE_LIBRARY_WARNING_THRESHOLD", 5)
+    library = _library_with_n_entries(5)
+
+    with tempfile.NamedTemporaryFile(mode="w", suffix=".bib", delete=False) as f:
+        temp_path = f.name
+    try:
+        with caplog.at_level(logging.WARNING, logger="bibtexparser.entrypoint"):
+            write_file(temp_path, library)
+    finally:
+        os.unlink(temp_path)
+
+    assert caplog.text.count("Writing a library with 5 blocks") == 1
+
+
+def test_write_string_does_not_warn_below_threshold(monkeypatch, caplog):
+    monkeypatch.setattr("bibtexparser.entrypoint.LARGE_LIBRARY_WARNING_THRESHOLD", 5)
+    library = _library_with_n_entries(4)
+
+    with caplog.at_level(logging.WARNING, logger="bibtexparser.entrypoint"):
+        write_string(library)
+
+    assert "Writing a library" not in caplog.text
+
+
+def test_write_string_does_not_warn_with_inplace_stack(monkeypatch, caplog):
+    """The suggested workaround must itself not warn, and must produce identical output."""
+    expected = write_string(_library_with_n_entries(5))
+    monkeypatch.setattr("bibtexparser.entrypoint.LARGE_LIBRARY_WARNING_THRESHOLD", 5)
+
+    with caplog.at_level(logging.WARNING, logger="bibtexparser.entrypoint"):
+        actual = write_string(
+            _library_with_n_entries(5),
+            unparse_stack=default_unparse_stack(allow_inplace_modification=True),
+        )
+
+    assert "Writing a library" not in caplog.text
+    assert actual == expected