diff --git a/sshfs/file.py b/sshfs/file.py
index 349f64d..a6711c1 100644
--- a/sshfs/file.py
+++ b/sshfs/file.py
@@ -26,32 +26,37 @@ def __init__(
         self.mode = mode
         self.max_requests = max_requests or _MAX_SFTP_REQUESTS
 
-        if block_size is None:
-            # "The OpenSSH SFTP server will close the connection
-            # if it receives a message larger than 256 KB, and
-            # limits read requests to returning no more than
-            # 64 KB."
-            #
-            # We are going to use the maximum block_size possible
-            # with a 16KB margin (so instead of sending 256 KB data,
-            # we'll send 240 KB + headers for write requests)
-
-            if self.readable():
-                block_size = READ_BLOCK_SIZE
-            else:
-                block_size = WRITE_BLOCK_SIZE
-
         # The blocksize is often used with constructs like
         # shutil.copyfileobj(src, dst, length=file.blocksize) and since we are
         # using pipelining, we are going to reflect the total size rather than
         # a size of chunk to our limits.
-        self.blocksize = block_size * self.max_requests
+        self.blocksize = (
+            None if block_size is None else block_size * self.max_requests
+        )
 
         self.kwargs = kwargs
 
         self._file = sync(self.loop, self._open_file)
         self._closed = False
 
+    def _determine_block_size(self, channel):
+        # Use the asyncssh block sizes to ensure the best performance.
+        limits = getattr(channel, "limits", None)
+        if limits:
+            if self.readable():
+                return limits.max_read_len
+            return limits.max_write_len
+
+        # "The OpenSSH SFTP server will close the connection
+        # if it receives a message larger than 256 KB, and
+        # limits read requests to returning no more than
+        # 64 KB."
+        #
+        # We are going to use the maximum block_size possible
+        # with a 16KB margin (so instead of sending 256 KB data,
+        # we'll send 240 KB + headers for write requests)
+        return READ_BLOCK_SIZE if self.readable() else WRITE_BLOCK_SIZE
+
     @wrap_exceptions
     async def _open_file(self):
         # TODO: this needs to keep a reference to the
@@ -61,6 +66,10 @@ async def _open_file(self):
         # it's operations but the pool it thinking this
         # channel is freed.
         async with self.fs._pool.get() as channel:
+            if self.blocksize is None:
+                self.blocksize = (
+                    self._determine_block_size(channel) * self.max_requests
+                )
             return await channel.open(
                 self.path,
                 self.mode,
@@ -81,6 +90,9 @@ async def _open_file(self):
     def readable(self):
         return "r" in self.mode
 
+    def seekable(self):
+        return "r" in self.mode or "w" in self.mode
+
     def writable(self):
         return not self.readable()