How specifically do custom fields affect performance? If they are configured as being available for all projects and issue types will they be evaluated each time we view and issue even though they were never placed on the screen and hence will never have a value? Does carefully restricting the issue types and projects that a custom field is configured for mitigate performance concerns to some extent? I noticed when doing a simple api call to get an issue it does appear to evaluate all the custom fields that the issue type and project is eligible for even if the custom field is not on any screen for that particular issue.
Before reviewing our custom field setup I was hoping to get a better understanding of exactly what it is about the custom field that makes it cause performance issues. We are only at about 250 custom fields so not a huge number but would like to ensure we do not put ourselves in a bad position.