I have finally installed it on my iMac 5K (i5, M290X). I can happily say that the lag is gone.
Here's what I did:
I opened several safari tabs, Photoshop images, a reference app (PureRef) containing several large images, a PDF file, Evernote, Notability, Tweetbot, Fantastical 2, Wunderlist, Pages, App Store and Finder. I then tried Mission Control: it was perfectly smooth. No dropped frames, responsive, beautiful.
I activated quick look on a 5K image (this caused the 'zoom' animation to be choppy on Yosemite) - the animation was fluid, zoom vas fast and instant.
I opened an image heavy PDF that struggled a bit on Yosemite - this time it was really, really smooth.
Every other aspect is good too, apps load fast, swiping between desktops is fast and, honestly, the whole system somehow emanates with speed and confidence. I'm still almost expecting it to lag or be choppy, but I just can't get it to falter. Even the little things, like, opening Dictionary definitions seems faster.
And this is DP1!!
So, in conclusion: there is NO UI lag on iMac 5K i5/M290X on El Capitan. Everything is smooth and responsive. PDFs are immediate and enjoyable to read.
When developers start implementing Metal into their apps (can't wait for the next Adobe CC release) - it will be even more amazing. The iMac 5K now feels like the supercomputer it was meant to be. I couldn't be happier with it on El Capitan.