From the manual:
SYSTEM REQUIREMENTS
...
Recommendations:
128MB or more of system memory. Maximum performance can be attained by setting Q3A's com_hunkmegs to two-thirds of your available system memory. Normal com_hunkmegs values range from 90 to 128; higher values are not needed but won?t hurt either.
...
RUNNING GENERATIONS ARENA
It is recommended to use a shortcut to start Generations Arena as opposed to selecting it from the Mods menu under Quake3. If you do not have a shortcut then create one using the following arguments:
"C:\Program Files\Quake III Arena\quake3.exe" +set com_hunkmegs 128 +set fs_game generations
Substitute the appropriate drive letter and Quake 3 folder for your installation. Mac and Linux users should alter the arguments appropriately. Consult your Q3A documentation.