Some general tips to improve the overall performance of the app:

• Use fast (Class 10 or U3) external memory cards
• Disable any energy saving of your device and enable performance mode
• Close background services if possible
• Try to keep your device cool to prevent downclocking of the cpu

How to improve the speed of the gallery:

(Settings can be found under Menu -> Preferences -> Performance)

• Enable "Use embedded exif thumbnails"
• Disable "Rotate thumbnails"
• Disable "Raw-Decode thumbnails"
• Disable "Render XMP Settings" (Preferences -> Displaying)

• Disable "Always Full-Size Preview" (Preferences -> Performance)
• Add folders you want to view to the library

• Set a lower Preview-Quality (Heading "Preview")
• Disable the histogram
• Export images in smaller sizes
• (Exporting only) Set a lower raw-decoder quality (Preferences -> Decoder)