How do I fix lag on mc without upgrading the computer?

Note: this is likely not server related, but mainly computer related...

(Here's the story)
I'll keep this nice and simple:
My family has two computers. One is a lenovo s30 workstation (handles Minecraft fine), and the other is a custom built computer thats been around since 2005. (Heavily upgraded)

I use the custom built one. Last year during the summer, I used to play Minecraft (1.7) for about a 3 hours each day, and experienced no lag.

Then when 1.8 came out in August, my fps dropped significantly on most servers, and is still laggy now. Then, I also used the Lenovo computer more.

Now, I've started to use the lenovo computer less, and used the custom built one more. But it lags badly. (Around 10 fps on medium settings)
Any ideas how I could fix lag? Optifine perhaps?
Buying a new computer or upgrading is a very very very last resort.

Sorry if this isn't clear, I'll clarify on comments. Title says it all, basically...

 

Hm could it be your internet? Anyone downloading videos or on netflix?

 

Nothing much has changed. We still have the same amount of devices. Same ISP. Minecraft does about 15 fps in single player, on a new world.

 

That is so.. weird, What windows do you have? Windows 7 or 8?

 

Try using optifine, closing anything you aren't currently using, and set java to a higher priority in the task manager (not sure if that last one helps much, I always thought it did)

 

Windows 7...

I'll give optifine a try... Looks like that's the last thing I could try.

Already on lowest.

 

Try this program it will remove the junk off the computer and speed it up.
(100% safe I use it!)

http://www.wikihow.com/Use-CCleaner

 

try increasing ram given to minecraft, update java, update drivers,

has your computer experienced overheating to the cpu/gpu, is your ram in single channel or dual channel (if two ram sticks) has your HDD gone through any sort of shock?

 

Your computer might be lagging because you might have bad stuff on it. I recommend that you get malware bytes.