Only just looked at this thread but agree with others you probably need to add memory.
I updated a late 2008 13" Macbook to Yosemite in February, and it was hopeless - just as you describe - with its original 2GB of RAM, no idea why Apple says Yosemite can run on 2GB. This Macbook was originally fitted with 2 x 1GB, no spare slots, so I bought 2 x 2GB, total 4GB, from Crucial to replace the original memory, total cost incl. postage: £33.59. Quick delivery.
has useful tools to advise and/or scan your system in respect of your options. I used their scan tool but you can also phone for advice.
It's a bit fiddly to replace memory in a Macbook, carefully follow instructions. Yosemite is now running OK for most purposes, but would be optimal with more than 4GB in total, if you have the option for more.
From this experience I decided to double the memory in a 2011 iMac when upgrading to Yosemite, from 4GB to 8GB - original specification was 2 x 2GB and 2 empty slots, so bought 2 x 2GB more for the iMac. Same cost for 4GB, £33.59 incl. postage, but a slightly different spec - use the tool on the Crucial site above to determine the precise parts to order. Fitting new RAM to an iMac is easy using instructions, and with a total 8GB it's been fine.
Edited by s_h (Sun 27-Sep-15 16:19:31)