LVM operates below the filesystem, so whatever it does, it does so at the disk level. So yes, indeed, when LVM implements encryption this is "full-disk encryption" (or, more accurately, "full-partition encryption").
Applying encrypted LVM is fast when it is done upon creation: since the initial contents of the partition are ignored, they are not encrypted; only new data will be encrypted as it is written. However, when applying encryption on an existing volume (as is typical of TrueCrypt) requires reading, encrypting and writing back all used data sectors; this includes sectors which were previously in use, even if they are not in use right now, because they may contain excerpts of some files which were later on copied around. So that kind of after-the-fact application of encryption requires reading and rewriting the whole volume. A mechanical harddisk will run at about 100 MB/s, so a 1 TB volume will need 6 hours (3 for reading, 3 for writing).
The encryption itself needs not be slow, at least if it has been properly implemented. A basic PC will be able to encrypt data at more than 100 MB/s, with AES, using a single core (my underpowered laptop achieves 120 MB/s); with recent x86 cores offering the AES-NI instructions, 1 GB/s is reachable. Thus, the CPU can keep pace with the disk, and, most of the time, the user will not notice any slowdown.
Of course, if you do "something crazy" like cascading algorithms, well, you've done something crazy and you will have to pay for it. Cascading three algorithms means having to compute all three whenever you read or write data. AES is fast; Serpent not so (about twice slower). In any case, cascading encryption algorithms is not a very rational idea. By default, "encrypted LVM volume" in Linux will rely on dm-crypt, which is configurable (several algorithms are supported) but does not indulge into voodooism cascades, and that's a blessing. (This does show one of the little paradoxes of security: if it is too transparent and efficient, then people get nervous. For the same reason, medicine pills must taste foul.)
