I am currently using Ubuntu 15.04, and have an enormous issue with lag. It is very laggy when I log on, and it stays laggy. 16.04 requires a 2GHz+ processor but I have a 1.4GHz processor. I checked the ram clock speed for my computer, only to find it being 599MHz, for some reason. I will try to remove a ramstick so that I can see a difference, and when I do I will answer this question, because before my PC was alot faster than this.
DDR RAM (double data rate) may be reported as half it's frequency. This is normal, just multiply the speed by two to get it's true frequency.
As for lag, the best solution I can offer would be to change your desktop environment to something more lightweight. Hopefully you can find an environment that has a feature set you enjoy, while being light enough to work well with your computer's abilities.