Well you just need to close down your internet C:
I use firefox so once it's closed I just press ctrl + alt + delete -> Processes -> then delete firefox.
THEN
I restart firefox xDD.
my computer doesn't lag too much.
except when i don't turn it off and i come back hours later. xD
then it's like, "what the hell!? this is your punishment for leaving me on!" xDD
I haven't found that my firefox starts to lag after having it on for a while. When it seems to lag is in the turning the computer on in the first place.