diff --git a/spacy/cli/convert.py b/spacy/cli/convert.py
index 8413c639b..2e52bfa41 100644
--- a/spacy/cli/convert.py
+++ b/spacy/cli/convert.py
@@ -5,6 +5,7 @@ from wasabi import Printer
 import srsly
 import re
 import sys
+import itertools
 
 from ._util import app, Arg, Opt
 from ..training import docs_to_json
@@ -130,15 +131,16 @@ def convert(
         )
         doc_files.append((input_loc, docs))
     if concatenate:
-        all_docs = []
-        for _, docs in doc_files:
-            all_docs.extend(docs)
+        all_docs = itertools.chain.from_iterable([docs for _, docs in doc_files])
         doc_files = [(input_path, all_docs)]
     for input_loc, docs in doc_files:
         if file_type == "json":
             data = [docs_to_json(docs)]
+            len_docs = len(data)
         else:
-            data = DocBin(docs=docs, store_user_data=True).to_bytes()
+            db = DocBin(docs=docs, store_user_data=True)
+            len_docs = len(db)
+            data = db.to_bytes()
         if output_dir == "-":
             _print_docs_to_stdout(data, file_type)
         else:
@@ -149,7 +151,7 @@ def convert(
                 output_file = Path(output_dir) / input_loc.parts[-1]
                 output_file = output_file.with_suffix(f".{file_type}")
             _write_docs_to_file(data, output_file, file_type)
-            msg.good(f"Generated output file ({len(docs)} documents): {output_file}")
+            msg.good(f"Generated output file ({len_docs} documents): {output_file}")
 
 
 def _print_docs_to_stdout(data: Any, output_type: str) -> None:
diff --git a/spacy/tests/test_cli.py b/spacy/tests/test_cli.py
index 745e1ac23..59c2e14ec 100644
--- a/spacy/tests/test_cli.py
+++ b/spacy/tests/test_cli.py
@@ -24,7 +24,7 @@ def test_cli_converters_conllu_to_docs():
         "4\tavstår\tavstå\tVERB\t_\tMood=Ind|Tense=Pres|VerbForm=Fin\t0\troot\t_\tO",
     ]
     input_data = "\n".join(lines)
-    converted_docs = conllu_to_docs(input_data, n_sents=1)
+    converted_docs = list(conllu_to_docs(input_data, n_sents=1))
     assert len(converted_docs) == 1
     converted = [docs_to_json(converted_docs)]
     assert converted[0]["id"] == 0
@@ -65,8 +65,8 @@ def test_cli_converters_conllu_to_docs():
 )
 def test_cli_converters_conllu_to_docs_name_ner_map(lines):
     input_data = "\n".join(lines)
-    converted_docs = conllu_to_docs(
-        input_data, n_sents=1, ner_map={"PER": "PERSON", "BAD": ""}
+    converted_docs = list(
+        conllu_to_docs(input_data, n_sents=1, ner_map={"PER": "PERSON", "BAD": ""})
     )
     assert len(converted_docs) == 1
     converted = [docs_to_json(converted_docs)]
@@ -99,8 +99,10 @@ def test_cli_converters_conllu_to_docs_subtokens():
         "5\t.\t$.\tPUNCT\t_\t_\t4\tpunct\t_\tname=O",
     ]
     input_data = "\n".join(lines)
-    converted_docs = conllu_to_docs(
-        input_data, n_sents=1, merge_subtokens=True, append_morphology=True
+    converted_docs = list(
+        conllu_to_docs(
+            input_data, n_sents=1, merge_subtokens=True, append_morphology=True
+        )
     )
     assert len(converted_docs) == 1
     converted = [docs_to_json(converted_docs)]
@@ -145,7 +147,7 @@ def test_cli_converters_iob_to_docs():
         "I|PRP|O like|VBP|O London|NNP|B-GPE and|CC|O New|NNP|B-GPE York|NNP|I-GPE City|NNP|I-GPE .|.|O",
     ]
     input_data = "\n".join(lines)
-    converted_docs = iob_to_docs(input_data, n_sents=10)
+    converted_docs = list(iob_to_docs(input_data, n_sents=10))
     assert len(converted_docs) == 1
     converted = docs_to_json(converted_docs)
     assert converted["id"] == 0
@@ -212,7 +214,7 @@ def test_cli_converters_conll_ner_to_docs():
         ".\t.\t_\tO",
     ]
     input_data = "\n".join(lines)
-    converted_docs = conll_ner_to_docs(input_data, n_sents=10)
+    converted_docs = list(conll_ner_to_docs(input_data, n_sents=10))
     assert len(converted_docs) == 1
     converted = docs_to_json(converted_docs)
     assert converted["id"] == 0
diff --git a/spacy/tests/training/test_training.py b/spacy/tests/training/test_training.py
index 25c55e12c..2e83580b5 100644
--- a/spacy/tests/training/test_training.py
+++ b/spacy/tests/training/test_training.py
@@ -195,7 +195,7 @@ def test_json_to_docs_no_ner(en_vocab):
             ],
         }
     ]
-    docs = json_to_docs(data)
+    docs = list(json_to_docs(data))
     assert len(docs) == 1
     for doc in docs:
         assert not doc.has_annotation("ENT_IOB")
diff --git a/spacy/training/converters/conll_ner_to_docs.py b/spacy/training/converters/conll_ner_to_docs.py
index c01686aee..8c1bad9ea 100644
--- a/spacy/training/converters/conll_ner_to_docs.py
+++ b/spacy/training/converters/conll_ner_to_docs.py
@@ -87,7 +87,6 @@ def conll_ner_to_docs(
         nlp = load_model(model)
     else:
         nlp = get_lang_class("xx")()
-    output_docs = []
     for conll_doc in input_data.strip().split(doc_delimiter):
         conll_doc = conll_doc.strip()
         if not conll_doc:
@@ -116,8 +115,7 @@ def conll_ner_to_docs(
             token.is_sent_start = sent_starts[i]
         entities = tags_to_entities(biluo_tags)
         doc.ents = [Span(doc, start=s, end=e + 1, label=L) for L, s, e in entities]
-        output_docs.append(doc)
-    return output_docs
+        yield doc
 
 
 def segment_sents_and_docs(doc, n_sents, doc_delimiter, model=None, msg=None):
diff --git a/spacy/training/converters/conllu_to_docs.py b/spacy/training/converters/conllu_to_docs.py
index 2e6084ae5..356021a1d 100644
--- a/spacy/training/converters/conllu_to_docs.py
+++ b/spacy/training/converters/conllu_to_docs.py
@@ -34,16 +34,14 @@ def conllu_to_docs(
         ner_map=ner_map,
         merge_subtokens=merge_subtokens,
     )
-    docs = []
     sent_docs_to_merge = []
     for sent_doc in sent_docs:
         sent_docs_to_merge.append(sent_doc)
         if len(sent_docs_to_merge) % n_sents == 0:
-            docs.append(Doc.from_docs(sent_docs_to_merge))
+            yield Doc.from_docs(sent_docs_to_merge)
             sent_docs_to_merge = []
     if sent_docs_to_merge:
-        docs.append(Doc.from_docs(sent_docs_to_merge))
-    return docs
+        yield Doc.from_docs(sent_docs_to_merge)
 
 
 def has_ner(input_data, ner_tag_pattern):
diff --git a/spacy/training/converters/iob_to_docs.py b/spacy/training/converters/iob_to_docs.py
index a2185fef7..60fb7df61 100644
--- a/spacy/training/converters/iob_to_docs.py
+++ b/spacy/training/converters/iob_to_docs.py
@@ -24,12 +24,10 @@ def iob_to_docs(input_data, n_sents=10, no_print=False, *args, **kwargs):
     msg = Printer(no_print=no_print)
     if n_sents > 0:
         n_sents_info(msg, n_sents)
-    docs = read_iob(input_data.split("\n"), vocab, n_sents)
-    return docs
+    yield from read_iob(input_data.split("\n"), vocab, n_sents)
 
 
 def read_iob(raw_sents, vocab, n_sents):
-    docs = []
     for group in minibatch(raw_sents, size=n_sents):
         tokens = []
         words = []
@@ -61,5 +59,4 @@ def read_iob(raw_sents, vocab, n_sents):
         biluo = iob_to_biluo(iob)
         entities = tags_to_entities(biluo)
         doc.ents = [Span(doc, start=s, end=e + 1, label=L) for (L, s, e) in entities]
-        docs.append(doc)
-    return docs
+        yield doc
diff --git a/spacy/training/converters/json_to_docs.py b/spacy/training/converters/json_to_docs.py
index d7df1d6f9..4123839f2 100644
--- a/spacy/training/converters/json_to_docs.py
+++ b/spacy/training/converters/json_to_docs.py
@@ -12,11 +12,9 @@ def json_to_docs(input_data, model=None, **kwargs):
         if not isinstance(input_data, str):
             input_data = srsly.json_dumps(input_data)
         input_data = input_data.encode("utf8")
-    docs = []
     for json_doc in json_iterate(input_data):
         for json_para in json_to_annotations(json_doc):
             example_dict = _fix_legacy_dict_data(json_para)
             tok_dict, doc_dict = _parse_example_dict_data(example_dict)
             doc = annotations_to_doc(nlp.vocab, tok_dict, doc_dict)
-            docs.append(doc)
-    return docs
+            yield doc