stimpy - I'm sure your SSD is running fast under XP, but it is only a matter of time until degradation kicks in. The SSD will run at full speed -UNTIL- it runs into blocks which have been deleted but not formatted. This depends on the size of your SSD and how much you've written to it

There's a quite good article describing the effect at: http://www.anandtech.com/show/2738/8

Although you need to know a couple of background facts before the article makes sense:

1. Before writing data to NAND the block must be written with all 1s. After the write cycle some wells are changed to 0s and this represents your data.

2. One Logical Block may contain pieces of multiple files. What this means is if you want to change something on that block you have to a) read the block, b) write all 1s to the block, c) write the new, changed block. You can't simply overwrite it without the format step.

3. SSDs have a limited number of write cycles per block before they fail. Today's MLC NAND is usually rated for about 3000-5000 writes. Although this doesn't seem like much wear-leveling algorithms spread the data around the SSD so one spot shouldn't fail well before the others.

When you delete a file in XP it just erases the entries from the directory & file allocation table, the actual data sectors are unchanged. This isn't so good with SSDs as if the SSD knows the block is formatted it doesn't have to do that 3-step process to rewrite which causes the slowdown. One downside (or upside?) of TRIM is that file undelete programs will probably not work.

Do follow gfunkdave's advice and never defrag a SSD, there is no rotational advantage to be gained.

Anyway an analogy for using TRIM would be "why do I need to change the oil in my five year old car, it's always run fine." which is true, but eventually something nasty will occur and rather quickly.