Mirek,

1) It's 3-5% as reported by KDE's system monitor which I'm assuming is an aggregate based on all cores and doesn't sample as frequently. On a single core it's closer to 60-90%. On top it becomes the #1 spot on the process list by far - nothing else uses that much CPU that frequently. Yes, other apps do spike but they spikes to more reasonable numbers. Example: firefox spikes to 13-25% while idle, plasma remains around 13-14% because it's never really idle, thunderbird spikes to around 13% (all of these are on top). The case could be easily made that theide is an outlier in this regard.

2) Yes, when I run using INotify the amount of CPU used drops considerably (top shows 0.7% at idle) and even libclang runs better. I get near instant popups on autocomplete whereas before it could sit there for a couple seconds waiting for them to show up.

I think it's worth investigating.