If I've got my torrent client up downloading shit, which is basically all the time, yes. It's not a hardware issue. Getting a new computer won't fix the problem. They don't 'slow down the computer' per se - they merely take noticeably longer to load, particularly if your internet connection is already under strain from something else - and they only exist because people are lazy and can't switch VLC's default screenshot image format to jpg, which is literally 3 or 4 mouse clicks.
Regular high res pictures? png is fine. Preferred even, depending.
Screenshots? png is stupid. It's taking a lossless image of an already compressed file. It's redundant, only practiced by lazy people, and very irritating to view if you have other crap siphoning off your bandwidth.
I probably got trolled just now, but some people seriously don't know why png screenshots irritate people.
For the rest of you, now you know.