The three most influential factors for Eclipse speed are:
using the latest version of Eclipse (3.4 Ganimede at the time of writing in 2008)
Note that David Balaic's comment (July 2014) contradicts that criteria which was working 6 years ago:
The "same" workspace in Indigo (3.7.2) SR2 loads in 4 seconds, in Kepler SR2 (4.3.2) in 7 seconds and in Luna (4.4.0) in 10 seconds. All are Java EE bundles. Newer versions have more bundled plugins, but still the trend is obvious. (by "same" workspace I mean: same (additionally installed) plugins used, same projects checked out from version control)
launching it with the latest JDK (1.7 at the time of writing, which does not prevent you to compile in your Eclipse project with any other JDK you want: 1.4.2, 1.5, 1.6 older...)
-vm jdk1.6.0_10\jre\bin\client\jvm.dll
configuring the eclipse.ini (see this question for a complete eclipse.ini)
-Xms128m
-Xmx384m
-XX:MaxPermSize=128m # NOTE: this option is obsolete in Java 8, it will be ignored
-Xss2m
[...]
The Xmx argument is the amount of memory Eclipse will get (in simple terms). With -Xmx384m, it gets 384MB of RAM, with -Xmx4G it gets 4GB, etc.
Note:
referring to the jvm.dll has advantages:
Splash screen coming up sooner.
Eclipse.exe in the process list instead of java.exe.
Firewalls: Eclipse wants access to the Internet instead of java.
Window management branding issues, especially on Windows and Mac.
But it can also have some drawbacks if you try to push the memory too high.
The default memory taken by Eclipse is the combination of MaxPermSize and Xmx. Here up to 512 MB total, which is quite enough for a 1 GB memory computer.