Since you are using the Page Cache plugin, it doesn't really matter. Since this plugin creates a static file of the whole page, the other cache mechanism is not triggered at all. Only if the plugin is not used (e.g. when cache was purged), then it would make a (slightly, barely noticeable) performance difference.
Sorry, I think the attachment functionality does not work currently after an update of the forum anymore... I have to check it when I have more time. In the meantime, could you please upload the screenshot to your server and post the link to this image? Thanks!
you can achieve it easily with the help of a language override:
Go to Extensions - Langauge(s) - Overrides. Select your language on the left side (e.g. English (en-GB) - Site). Click on the New button, use the constant PLG_EASYCONTENTRESTRICTION_PASSWORD_REQUEST and define your own text.