Unity uses a lot of CPU resources. You may look to a method of removing Unity and using gnome. Of course, if you remove a lot of the defaults of the distro you might lose some support for other functions that might be associated with the distro.
I understand a lot of public sentiment that lots of wasteful resources are built into the various OS' each day. The developers try to make things convenient for the users and add features. These features take up CPU cycles. That's why XP was slower than Windows 3.1 and Windows 3.1 was slower than DOS. Each generation required more resources from the computer. If Windows XP could run on one of the earlier computers (a 286 for example) because of the CPU cycles, it would probably take nearly a year for it to boot up. It would probably also take a week for a mouse movement position to be updated.
Please consider that hard drive space of a new computer is measured in 100's of gigs rather than 100's of megs as when XP was release. Of course when DOS first came out the hard drive sizes were measured in Mbs (not 10's of Mbs). Thge first hard drive I bought was actully 2 megs for two dollars.
The CPU speed of the computers running DOS were less than 1MHz. The CPU speeds of computers today is measured in GHz... still in the single digits, but imagine what the applications will be doing when the hard drive space and speed is 1000 times what they are today and the CPU's are measured in triple digits of GHz. The developers will add even more functionality that will take even more cycles.
I believe the best suggestion for you is to look for a Linux distribution that requires less resources and has less default functionality, or get a higher performance computer, or deal with the slower operation.
As computers develop in performance, the developers are going to put the CPU cycles to use... not only the developer for the OS, but the developers of each of the application that you load to the computer.
I understand my answer might not be the best and the most direct answer, but hopefully it might ease some of your aggravation and get you quicker to the resolution of changing distros if Ubuntu isn't for you. Looking for an older version might cause you to lose support from the developers.
But you can always pick the oldest version of Ubuntu that is currently supported. The performance would most likely be higher, because it'll actually do less in the background.