I commonly assign modules to template position none. In my index.php template, such a position doesn't exist - so the modules don't show. On the pages, I want them to show I use: {loadmoduleid 100}.
Is there a performance impact on displaying modules on every page? Or should I only set it to only the specific pages it appears on? I'm using advanced module manager.
There is no simple answer to this as exact performance would depend on a number of factors, including number of modules, menu items and installed plugins. The important thing to understand is that modules are not executed until actually needed. I.e. if module is assigned to a position not present in the template, it will not be rendered. So there is at least no major performance impact. Plugins that heavily process modules during onAfterModuleList event could affect performance though.
There is also a trade off when using menu assignments. This is because for every menu item assigned/unassigned a database entry is made in #__modules_menu table. This could affect performance when loading the initial module list. And when using "On all pages except those selected" assignment mode, plugins with onAfterModuleList could be greatly affected since the module list contains duplicate instances of the same module, one for each menu item unassigned.
templateDetails.xml have no impact here, they're only used in module editing form. Positions need to be present in the template file (e.g. index.php). Or rendered manually like by using the plugin.