关于TCP传文件的一个疑惑？

我在传送文件的时候，中途就会出现“传输异常终止”。只有在加入 sleep语句后，文件才能顺利传输（不计大小）。
但是这样会影响传输速度。
怎样才能在避免影响速度的情况下顺利传输文件？（最好像飞鸽那样的性能）
一下是代码//服务器端传文件函数
procedure TForm1.TCPTransFile(Socket: TSocket;filename:String);
Const   BlockLen=1024*4;
var     Ftrans:File of Byte;
        Flen:Integer;
        BlockNum,RemainLen:Integer;
        BlockBuf:Array[0..BlockLen-1] of Byte;
        i:Integer;
        SendLen:Integer;
begin
        AssignFile(Ftrans,filename);
        Reset(Ftrans);
        Flen:=FileSize(Ftrans);
        BlockNum:=Flen div BlockLen;
        progressBar1.Max := BlockNum+1;
        RemainLen:=Flen mod BlockLen;
        SendLen:=1;
        for i:=0 to BlockNum-1 do
                begin
                if (SendLen<=0) then
                Break;
                BlockRead(Ftrans,BlockBuf[0],BlockLen);
                SendLen:=send(Socket,BlockBuf,BlockLen,0);
                sleep(1);   //设置延时
                progressBar1.Position := i;
                Application.ProcessMessages;
                end;        if (SendLen<=0) then
                begin
                CloseFile(Ftrans);
                TTY('传输异常终止！');
                Exit;
                end;        if RemainLen>0 then
        begin
        BlockRead(Ftrans,BlockBuf[0],RemainLen);
        SendLen:=send(Socket,BlockBuf,RemainLen,0);
                if (SendLen<=0) then
                begin
                CloseFile(Ftrans);
                TTY('传输异常终止！');
                Exit;
                end;
        end;
        CloseFile(Ftrans);
        TTY('传输完成！');
end;//客户端接受文件
procedure TForm1.TCPClientData(Socket: TSocket);
Const   BlockLen=1024*4;
var
       Ftrans:File of Byte;
       size,Recelen:Integer;
       BlockBuf:Array[0..BlockLen-1] of Byte;
begin   
       AssignFile(Ftrans,filename);
         Reset(Ftrans);
        //移动文件指针到文件末尾
         Seek(Ftrans,GetRecordCount(filename ));         Recelen:=recv(Socket,BlockBuf,BlockLen,0);         if (Recelen>0)then
         begin
            BlockWrite(Ftrans,BlockBuf[0],Recelen);
            Application.ProcessMessages;
         end;
         CloseFile(Ftrans);
     end;