- Use a class 10 SD card for best speed. The USB bus can't come much higher than 30MB/s so you don't have to buy any extremely fast ones though. Not all cards are compatible, check the compatibility list: http://elinux.org/RPi_SD_cards
- Use the HardFloat version of Raspbian instead of the SoftFloat. HF has much faster floating point operations - however SF is required for running Java. So it's either Java or performance, like normal.
- The official Raspbian image gives low network speeds: http://elinux.org/RPi_Performance#NIC
- A graphics driver by Simon / teh_orph is using hardware acceleration for some instructions: http://www.raspberrypi.org/phpBB3/viewtopic.php?f=63&t=28294 installation instructions: http://elinux.org/RPi_Xorg_rpi_Driver
- The firmware can be upgraded which gives, among other things, better GPU performance. https://github.com/Hexxeh/rpi-update
- Turbo mode has been introduced. http://www.raspberrypi.org/archives/2008
- You can tweak the split of RAM between GPU and CPU. Now also Dynamic Memory Split is supported: http://elinux.org/RPiconfig
- X performance will increase if you remove the background image
- Recent versions of NodeJS have been optimized to utilize hardfloat (hardware float operations). Don't use apt-get, instead use nvm to get a more recent version of NodeJS.
- If you use libraries or frameworks like Express, turn off logging. SD I/O is slow.
- HW acceleration for CSS3 and HTML5 could be possible with Simon's accelerated X driver. Iceweasel (Debian's version of Firefox) will probably be better supported than Chromium (the non-Google distribution of Chrome) in the start, since its instructions are easier to optimize according to Simon / teh_orph. Midori which is already installed is based on WebKit and more lightweight than Chromium.
However, all web browsers using X will probably never be impressively fast, since X is very hard to accelerate (The problem with X performance is mentioned in this thread http://www.raspberrypi.org/phpBB3/viewtopic.php?f=63&t=4649)