One of the things that screws up HDD performance much worse than placement of files on disk is randomness in the usage pattern. The mechanical nature of a HDD means that when you read and write lots of small files in different sectors, the head spends more time moving around than reading or writing. Back when we used to defragment Windows filesystems, we doing a bunch of up-front disk optimization to organise files into continuous chunks so they could be read back quickly when needed.
The biggest problem I have seen with these situations is that you don't have direct control over the order of operations that the disk will be asked to perform. You think that because your file is written contiguously that it will be read that way. But depending on how busy the system is, that might not be the case. Where many processes are contending for disk access, and especially when the kernel is doing a lot of swapping to the same device, that head might be racing back and forth regardless of your file placement, and your disk performance goes straight into the toilet.