I am running xubuntu for almost 5 years now, and i switched from few versions and previous versions of xubuntu were pretty fast but latest one 16.04 is kinda different, i recently switched from 14.04 to 16.04 on my laptop and with previous version my laptop was working pretty fine and fast.
Now i know it's not the newest high end laptop but this is pretty decent laptop and good configuration even for today requirements. I can play even latest games on it and windows is running very fast.
The laptop is Toshiba Qosmio F50-125 with specs
Intel Core 2 Duo P8600 processor clock speed : 2.40 GHz Front Side Bus : 1066 MHz 2nd level cache : 3 MB
4GB DDR2 RAM 800Mhz
Nvidia 9700 GTS 512 MB DDR3 dedicated ram
I have 120GB SSD as my main drive, and 400GB HDD from laptop i put it in DVD drive instead of DVD.
The problem is that i noticed that my CPU never gets used more than 70-80% and at that point my laptop starts running slow, the browsers like firefox, or chrome starts with lag and freezes from time to time, if i watch the movie i can't do anything else because it starts skipping frames, and i can't even try to load FHD video from youtube.
I have latest proprietary drivers from Nvidia. And i know that my laptop is capable pushing this all with no problem i could watch even 4k video on youtube on 14.04 xubuntu.
And also i taken system monitor to see the usage of pc. At the time i made a screenshoot you can see that i have a few services that can stress pc and use resources like xampp with mysql and apache running, radio tray which plays radio music, and few others. And also firefox of course and firefox was playing 720p video.
You can see that i have enough memory and also that my CPU is about 70% but i feel alot of lag on my video like it's on 20fps
I also checked my GPU use when watching video and it's not nearly to struggle, almost don't feel the video.