So. Matthias Fuchs of â€œSpeeding up KGet with Callgrindâ€ fame pointed out to me on IRC the other day that KIconLoader seemed to still be using a lot of time to load icon image data, even when the icon should have already been cached.
I managed to confirm the issue. Turns out that when I ported KIconLoader to use KSharedDataCache, I didn’t employ KIconLoader’s other cache (a pixmap cache ) in all the instances where it made sense. Specifically, the pixmap cache only had things added when an entry was also added to the KSharedDataCache.
But this doesn’t always make sense. The KSharedDataCache is shared across all KDE Platform-based processes, but there’s a different pixmap cache for every KIconLoader. So, it’s perfectly possible to have many processes never have to add an icon to the KSharedDataCache.
Now, new pixmaps that are read from a KSharedDataCache are added to a KIconLoader’s own personal pixmap cache to avoid trying to search the KSharedDataCache next time. This has the potential to significantly speedup icon-based operations, especially in situations where an application uses the same pixmaps over and over. This speedup is in trunk (future 4.6) and will also be released with KDE Platform 4.5.2.
Matthias (and Ingo KlÃ¶cker) provided another speedup, by converting some slow QString-based operations to use a QStringBuilder class introduced in Qt 4.6. This gave an 8% speedup on an icon microbenchmark, so not insignificant. This one will only be in KDE Platform 4.6 at this point, although I wouldn’t feel horrible if it were to be backported also.
Incidentally, this was the first I’d heard of regarding QStringBuilder, so it’s something I’d recommend brushing up on if you haven’t heard of it yet. The QStringBuilder documentation is part of the QString API reference (search for “More Efficient String Construction”).
 Every KIconLoader instance also has its own pixmap cache, which holds QPixmaps, while KIconLoader’s KSharedDataCache holds QImages.