It is an OLD i3 Dell from 6 years ago desktop.
I've been running it for ages on a mbp with 2.6GHz i5/8Gb RAM. It's lightning fast.
Very occasionally it will appear to be hogging CPU - typing is slow and it might hang for a couple of seconds when you're navigating around. This only seems to happen when I've had it running for days if not weeks without a restart. I just restart it and it's back to being lightning fast.
I really like Atom and would be pretty upset if it stopped working well for me, so would be curious to know what kinds of things people suspect cause it to have issues.
Is it just that my machine is powerful enough to not notice, and it's only really a problem on less powerful machines? I don't run many plugins, and installing many of these tend to bring it down? It runs well on OSX but not other platforms?
It's important for Atom's extensibility that these kinds of events are provided, because it allows many major editor features to be implemented as separate packages, but it means that naively-implemented packages can really slow things down.
Many Atom packages are pretty new, and their authors may not have put a lot of effort into optimization yet. In the past few months, the team has put a lot of effort into stabilizing and solidifying our APIs. We're hoping that now that the APIs are stable, the package ecosystem will really start to mature.
I suspect that people who complain about speed are either on lower end hardware or are comparing it directly to a native editor directly and consider any typing latency at all to be unacceptable.
Do you literally mean that it hangs for a couple of seconds? That seems like an eternity, especially if it happens during navigation.
 I know such a search is ridiculous, but both editors perform search-as-you-type, although atom does attempt to delay that if you type quickly enough. Anyway, it's just an example to demonstrate the speed difference.
The built in search is slow (for now), but they have been steadily making gains and use performance testing to measure progress.
Good news, it exists. If you know Go it's possible to see it sooner: