On the newer macs, AES is accelerated in the CPU. No problem there whatever you do.
On older ones, it is possible to notice a performance decrease, but it's unlikely you will if you have a traditional spinning rust hard drive-- even the oldest mac that can run Lion's CPU is fast enough to encrypt and decrypt many megabytes per second of AES-encrypted data. Most hard drives, especially those in laptops, are unable to keep up with it.
Of course, if you're doing something with a lot of data that also requires a lot of processing, you'll find it a bit slower.
Also, if you do have an SSD, there is one other thing to look out for. Some SSDs, notably those based on Sandforce controllers, get most of their speed out of a compression and deduplication-algorithm. This algorithm can not work if you're encrypting the data before it goes to the ssd's controller. This slowdown, I can tell you, is noticable. So if you intend to use FV and are going to get an SSD, get one that does not base its performance on compression.
You can tell if your CPU does AES-NI by using the command sysctl hw.optional.aes.