'''Often a slow computer is caused by a HD that is too full'''. In order for the OS to create/write VMEM and swap (temporary) files you boot volume must have 10% of it's total volume free (empty) for this purpose.
You can find out [http://www.thexlab.com/faqs/freeingspace.html|if your boot volume is too full], ''(if it's really full you get a error message at boot) '' and delete inconsequential data to create enough free space.
Another reason for a slow computers is a boot volume with permission or MBD problems. '''Booting to repair (R) and running Disk Tools'''->Repair permissions and Repair Disk can cure those issues.
'''Lastly if you did not opt out of full disk encryption '''at install of Yosemite that slowdown is the files being encrypted... it should be significantly faster over time if that was the cause.
''If this answer is acceptable '''please remember to return and mark it'''.''