Hi, a file is divided into 1024 segments (or less some times) before its sent to the data requester. Currently the network encrypts 1% of those segments so it would be around 10 segments to be encrypted using AES or Chacha depending on the hardware support.
The only overhead is the encryption of those 10 segments which is very small and normal in this days condisering most hardware of storage providers have AES instruction set on CPU, so to conclude there is a very small overhead which is completely fine.

We also randomize those segments, so the additional overhead is random seek for 1024 segments on the HDD/SSD which is again negligible.

You can verify these in: https://github.com/filefilego/filefilego/blob/master/common/file_utils.go