What are the pros and cons, performance-wise, of enabling and using FileVault? Is there any at all, or does it slow a computer down significantly or on the contrary speed it up? Also, what effect will it have on Time Machine-backups (performance-wise)?
(I'm assuming you are referring to FileVault 2 found in OS X Lion)
The folks at AnandTech have run some performance benchmarks on the new FileVault. Quote:
[emphasis above is mine]
So to answer your question:
Filevault 2 supports Intel® Advanced Encryption Standard (AES). It's important to know if your Mac has an Intel processor which supports this, as it significantly reduces the CPU overhead. Search for you CPU here an look if is supports the New AES instructions.
My Macbook Pro 2011 got this feature enabled via a firmware update (Apple did not enable this with the newly shipped MBP). With AES encryption acceleration, the performance drop with Filevault 2 enabled is almost unnoticeable during normal usage. The maximum read speeds of my SSD only dropped from 510MB/s to 490MB/s.
Ars Technica, published an extended review on Filvault 2 in Lion. They say:
As the overall CPU load is nearly unchanged, the battery life is not affected significantly.
Time Machine Backups
If you backup up wirelessly (unencrypted) or locally (encrypted, USB) the performance of Time Machine will not be affected significantly.
I've been using Filevault 2 encryption on a MBP 8,2 for three months.
This depends to a large extent on your hardware. Filevault essentially encompasses 2 major elements:
CPU bound, the faster your CPU, the quicker the OS can encrypt and decrypt the data on the disk. Filevault uses 256 bit encryption, so it's a fairly meaty process
Once stuff is encrypted (or before it can be decrypted) is needs to be read from or written to disk. The difference between an SSD and traditional spinning platter media is enormous. What might be a slow decryption process could be made up for using lightening fast access.
So, really it depends on all sorts of things, and how they can affect the above. You don't list hardware, so that makes it near impossible to gauge.
Anecdotally, as it's the only measure I have to hand, I have had Lion installed on a 1.6Ghz Macbook Air with SSD both with and without Filevault. In use it booted marginally slower, but I had to time it (I forget the timings, but it was of the 15% slower mark or so) but in use I could see no discernible difference.
Assuming you have a USB Time Machine disk, I would say the impact will be non-existant, as it's highly unlikely that any affect on performance would take it down to below your normal USB performance.