Performance

[Dead Eye]

Its more to do with where the data is stored on the hard drive (fragmentation)

Lots of tiny pieces of data (each post for example) requires a different read request if its not stored exactly next to another piece being requested (which is unlikely). Therefore you need an HDD with a very amount of IOPS more than transfer rates. This is where RAID arrays and SSDs come in to increase performance and yield higher IOPS to the OS / Applications

Its worth noting that having all the avatars in a single directory isn't going to make any difference to data fragmentation unless I'm missing something.