Slower than other file browsers ?
What other ? Konqueror ? Krusader ?
The Dolphin is the default KDE file manager but you don't need to use it. System Settings > Default Applications > File Manager.
:~$ apt-cache show kdegraphics-thumbnailers
Description: graphics file format thumbnailers for KDE SC
These plugins allow KDE software to create thumbnails for several
advanced graphic file formats (PS, RAW).
sudo apt-get install kdegraphics-thumbnailers
Dolphin > Settings > Configure Dolphin > General > Previews: Show previews for PostScript, PDF and DVI Files
You may need to rise the "Do not create previews for" limit !
--- EDIT ----
Slower than Nautilus, its GNOME brother.
I have not used the Nautilus but the developer /1/, /2/ is promising improved performance with the KDE 4.8 & Dolphin 2.0.
...Though I don't get why I have to hit the "preview" button on
toolbar for every directory I enter in. How can I make it the default
One way: Dolphin > View > Adjust View Properties. Press the F1 key, when the Dolphin is active, to get the Dolphin Handbook.
Another way /3/: Open the Dolphin in the ~/.kde/share/apps/dolphin/view_properties/global/ (*1) and turn the Icons/Details/Columns/Preview/... on/off. The settings are saved to the ~/.kde/share/apps/dolphin/view_properties/global/.directory file and used as the global defaults - IF the local folder does not have own .directory file with the overriding settings.
KDE .directory file:
The .directory file is a KDE folder view properties file. It contains the folder viewing settings for a directory in KDE. It includes settings such as icon positions and view mode (icon, list, etc.). Automatically created when the view settings of a folder are modified. Hidden by default.
(*1) If the ~/.kde/share/apps/dolphin/view_properties/global/ directory does not exist, you could make it.
More of the Dolphin: KDE UserBase /4/ and the Dolphin KDE Home /5/.