In v6.17, wheel scrolling of text windows is lagged, that is, there's some "residual scrolling" after the wheel is no longer moved. It's definitely not so in the snappy 6.16.
Another side effect is lagged window dragging. To be able to see it better, try dragging around an empty query window on an empty mIRC desktop, then dragging a full query window on some active channels. It changes from smooth to clearly lagged.
This was exhibited on a 550MHz Pentium 3. Perhaps these problem aren't apparent, or less so, on faster computers.