Well, if you played at high settings before it suggests something is wrong with your PC, not necessarily Major though.
Its worth using a program such as 'Defraggler' to defragment your harddrive, this will speed up your Harddrive.
Imagine, one cabinets is split in pieces, each laid next to each other, if you buy a table forr example hen, leave the bits apart, and systematically put them by each other, you are seeing what a harddrive was.
However, if you then want to add something to your table, all he gas are filled, so it has to go next to the table. If you then want to build it, you have to travel furher to get the bits, thus taking longer, same with a Harddrive.
Ensure that your fan is not clogged up with dust etc, as when a Graphics card gets hot, it often slows itself down to ensure it does not overheat.
Short Summary
1. Defragment PC using Defraggler or similar
2. Ensure fan/entrance is clean, relatively dust free.
3. Download the Standard Low Resolution pack as linked in the firs page of this topic, but the appropriate files where they're meant to be, theres a readme that comes with it.
4. Test to see if game has sped up sufficiently, if you are not happy then download the 'Lower quality Particle.txd' link on the first post of topic and put it in the 'models' folder in the San Andreas Directory.
It's important you get steps 3 and 4 in the right order, the game will still work if you don't, but the game wont speed up as effectively.