The dependence of the speed of the server on the number of files in a folder

Depends on the file system. For ext3/ext4 large number of files is not a problem.
Split necessary, since it will be more convenient bekapit, serve and develop.

The file system is loaded from the total number of files on disk(not much) and frequency of access, number of files in the folder..

Will definitely share it. With the growth of number of files in one directory and you will notice slow access to a specific file by its full path, create a few levels of subcategories, so as not to encounter this problem.
And on account of the control panel of the hoster, here the situation is slightly different. The panel scans the contents of your folders, given the number of files it takes some time.

You have to share, on a million when it polled the folder will be visible lag.

If I'm not mistaken, in UNIX systems everything is a file: directories, ports, devices, and so on. So she doesn't care. Files are stored on disk, regardless of the folders.
But I would recommend to separate folders. Imagine that you work(view, delete, etc.) with a list of files that are in the 50k+, for example.

We are in the project with a VERY large number of files, why it was divided into folders instead of the name abcdefgh => a/b/c/abcdefgh. Our CTO said that it is faster. Although I have not tested in practice.
\r
That checked, then go to the folder with the same MC with 10K files is not so simple...

I'm divided on the date.
/images/2010/1/
/images/2011/10/
etc. I'm afraid to count how many I have in there files, but vozmozhnosti terabyte screw for inode I has already exhausted 50%