Other Games Strange lag issues....

Am I the only one experiencing this, but, just read.
I use 3 computers on a normal basis, my laptop, a desktop fujistsu PC, and a desktop iMac.
But I've been having some pretty strange lag issues recently, and I was wondering if any of you knew what I can do to make it better.
I really only play 3 games very frequently, and they are Minecraft, osu!, and League of Legends.
The PC and laptop use adobe flash and I'm not sure exactly what the iMac uses.
Getting straight to the point....
The PC has absoloutley no lag whatsoever on osu!, I've got 127 FPS. It only runs 16 max on LoL, and 10 max on MC.
The iMac has lag issues on osu!, I can't really play osu! on the iMac. It seems to have close to no lag on LoL along with Minecraft.
The funny part is, my $500 Acer laptop from costco runs osu! and LoL perfectly. I've had it for 3 years now. My Laptop is OK with minecraft, but not as good as the iMac, about the same as the PC.
The PC actually used to do fine with LoL and not so great with osu!, then all of a sudden it switched the two. Now it doesn't do great with LoL, but perfectly with osu!.
The iMac hasn't really changed. My laptop used to be really slow, but I did a factory reset so now it's okay.
I just got the PC about a year ago, and I clear my cookies, cache, and all of that stuff frequently.
Does anyone know how I can fix this or what may have caused it in the first place?
I use firefox to browse the PC and laptop, safari for iMac.
The PC and laptop are both windows. My acer is windows home premium, I am unsure about the PC.
I know this is a huge paragraph, but I was just wondering, because hey, this is a gaming community, there has to be at least one stereotypically fufilling "nerd" here.