Read/Write speed of encrypted sparsebundle I'm in way over my head with even being on this website, let alone attempting to ask a question… but here's an attempt.  

Basically, I want to have a password-protected folder on my Mac which is resizable as needed (sparsebundle seems to be best).  It's recommended by Disk Utility to go with 128-bit AES because 256 is "slower".  

This got me thinking, how slow is 128?  Is this just while it's initially encrypting?  If so, I don't mind.  The problem is, I'll be storing session files for use with music software in this folder, which means constant reading and writing of files as I work.

Will the encryption slow me down?  I'm hoping that once I put my password in and "open it up", it "stays open" and read/write speeds are as if it isn't even encrypted, but I suspect this isn't the case.  I also don't want to create a speed bottleneck that renders my internal SSDs pointless.  Thanks so much for any thoughts on this situation, much appreciated.
It's recommended by Disk Utility to go with 128-bit AES because 256 is
  "slower"


Yes, AES-256 is 40% slower than AES-128, because it has 14 as opposed to 10 internal encryption rounds.


  This got me thinking, how slow is 128?


On a modern CPU, AES-256-CTR runs at about 1cpb (cycle per byte), meaning AES-128-CTR will roughly run at 0.75cpb (this assumes that the implementation is properly optimized). If you now have a CPU running at 3GHz you get about 4GB/s encryption throughput.


  Will the encryption slow me down?


That very much depends. It depends on the optimization quality of the AES implementation. It depends on the speed of your drive(s). It depends on the load on the CPU during the write / read operations except for the encryption. It depends on the specific CPU, ie whether it has ressources left for encryption.

Usual saying is that it will slow the PC down by a few per cent, so not much.


  Is this just while it's initially encrypting?


Initially it will encrypt the entire area you allocated so an attacker can't distinguish "empty space" from "meaningful data" when looking at it. During the normal usage, only the parts that need changing are changed, meaning you can use the actual size of the data you save / read for your timing calculations.