diff --git a/scrapy/commands/check.py b/scrapy/commands/check.py
index 8a3cf5078..56dc1ea55 100644
--- a/scrapy/commands/check.py
+++ b/scrapy/commands/check.py
@@ -81,14 +81,12 @@ class Command(ScrapyCommand):
         spider_loader = self.crawler_process.spider_loader
 
         async def start(self):
-            for request in conman.from_spider(self, self._result):
+            for request in conman.from_spider(self, result):
                 yield request
 
         with set_environ(SCRAPY_CHECK="true"):
             for spidername in args or spider_loader.list():
                 spidercls = spider_loader.load(spidername)
-
-                spidercls._result = result  # type: ignore[assignment,attr-defined,method-assign,return-value]
                 spidercls.start = start  # type: ignore[assignment,method-assign,return-value]
 
                 tested_methods = conman.tested_methods_from_spidercls(spidercls)
diff --git a/scrapy/core/engine.py b/scrapy/core/engine.py
index 1e8d01255..b08de6149 100644
--- a/scrapy/core/engine.py
+++ b/scrapy/core/engine.py
@@ -180,7 +180,12 @@ class ExecutionEngine:
     def unpause(self) -> None:
         self.paused = False
 
-    async def _process_next_spider_start_yield(self):
+    async def _process_start_next(self):
+        """Processes the next item or request from Spider.start().
+
+        If a request, it is scheduled. If an item, it is sent to item
+        pipelines.
+        """
         try:
             item_or_request = await self._start.__anext__()
         except StopAsyncIteration:
@@ -201,24 +206,28 @@ class ExecutionEngine:
 
     @deferred_f_from_coro_f
     async def start_request_processing(self) -> None:
-        """Process start items and requests in an asynchronous loop.
-
-        Items are scraped. Requests are scheduled.
-        """
+        """Starts consuming Spider.start() output and sending scheduled
+        requests."""
         if self._started_request_processing:
             raise RuntimeError("Request processing already started")
         self._started_request_processing = True
 
+        # Starts the processing of scheduled requests, as well as a periodic
+        # call to that processing method for scenarios where the scheduler
+        # reports having pending requests but returns none.
         assert self._slot is not None  # typing
         self._slot.nextcall.schedule()
         self._slot.heartbeat.start(self._SLOT_HEARTBEAT_INTERVAL)
+
         while self._start is not None:
-            await self._process_next_spider_start_yield()
+            await self._process_start_next()
             if not self.needs_backout():
+                # Give room for the outcome of self._process_start_next() to be
+                # processed before continuing with the next iteration.
                 self._slot.nextcall.schedule()
                 await self._slot.nextcall.wait()
 
-    def _start_next_requests(self) -> None:
+    def _start_scheduled_requests(self) -> None:
         if self._slot is None or self._slot.closing is not None or self.paused:
             return
 
@@ -403,7 +412,7 @@ class ExecutionEngine:
             raise RuntimeError(f"No free spider slot when opening {spider.name!r}")
         logger.info("Spider opened", extra={"spider": spider})
         self.spider = spider
-        nextcall = CallLaterOnce(self._start_next_requests)
+        nextcall = CallLaterOnce(self._start_scheduled_requests)
         scheduler = build_from_crawler(self.scheduler_cls, self.crawler)
         self._slot = _Slot(close_if_idle, nextcall, scheduler)
         self._start = await maybe_deferred_to_future(
diff --git a/scrapy/core/spidermw.py b/scrapy/core/spidermw.py
index bace52e4e..f58d96fd7 100644
--- a/scrapy/core/spidermw.py
+++ b/scrapy/core/spidermw.py
@@ -8,7 +8,7 @@ from __future__ import annotations
 
 import logging
 from collections.abc import AsyncIterable, Callable, Iterable
-from inspect import isasyncgenfunction, iscoroutine, iscoroutinefunction
+from inspect import isasyncgenfunction, iscoroutine
 from itertools import islice
 from typing import TYPE_CHECKING, Any, TypeVar, Union, cast
 from warnings import warn
@@ -387,20 +387,20 @@ class SpiderMiddlewareManager(MiddlewareManager):
         dfd2.addErrback(process_spider_exception)
         return dfd2
 
-    @inlineCallbacks
-    def process_start(
-        self, spider: Spider
-    ) -> Generator[Deferred[Any], Any, AsyncIterable[Any] | None]:
+    @deferred_f_from_coro_f
+    async def process_start(self, spider: Spider) -> AsyncIterable[Any] | None:
         self._check_deprecated_start_requests_use(spider)
         if self._use_start_requests:
             sync_start = iter(spider.start_requests())
-            sync_start = yield self._process_chain(
-                "process_start_requests", sync_start, spider
+            sync_start = await maybe_deferred_to_future(
+                self._process_chain("process_start_requests", sync_start, spider)
             )
             start = as_async_generator(sync_start)
         else:
-            start = yield self._iter_seeds(spider)
-            start = yield self._process_chain("process_start", start)
+            start = spider.start()
+            start = await maybe_deferred_to_future(
+                self._process_chain("process_start", start)
+            )
         return start
 
     def _check_deprecated_start_requests_use(self, spider: Spider):
@@ -473,14 +473,6 @@ class SpiderMiddlewareManager(MiddlewareManager):
                 f"https://docs.scrapy.org/en/VERSION/news.html"
             )
 
-    @staticmethod
-    def _iter_seeds(spider: Spider):
-        fn = spider.start
-        if isasyncgenfunction(fn):
-            return fn().__aiter__()
-        assert iscoroutinefunction(fn)
-        return deferred_from_coro(fn())
-
     # This method is only needed until _async compatibility methods are removed.
     @staticmethod
     def _get_async_method_pair(