When loading template and language files, SMF uses eval() instead of require_once()/include_once(). The specific reason for this is that, with eval(), if there is an error, PHP tells SMF. With include_once(), it just spits out an error.
Benchmarks show that eval() isn't much slower, and it's no more a security risk because we mean to include_once() the file anyway. However, if you have an accelerator installed, like eAccelerator, eval() is slower. Much slower. And it uses more memory, too.
As such, you'll want to run the following query to disable eval():
REPLACE INTO smf_settings
VALUES ('disableTemplateEval', '1');
Please note that this will also disable the "pretty" error messages that highlight the line the error occured on, too. All you get is PHP's error message, after this. Still, it's not a huge price to pay... since you can always change it back to 0 when you mess with your templates.