stingsstrings in a struct as values to static properties
This is a head splitting-splitting description. First, strings are always reference types, no matter how or where you declare them. Your struct will be empty because you have no non-static members. The references will take up 4 * 12 bytes in size. Where the characters are will no longer matter, they will remain in the same place for the lifetime of your process. If you want the text to be laid out in a struct you must use character arrays instead and declared them non-static.
Second, there are no properties in your example. You only have data members.
Performance-wise you gain nothing by having strings contained in a struct. You may even lose performance if you are not careful by cause boxing and unboxing.
Either way, for accessing the strings it would not bear significance. Reading a string of only a couple of characters will already take longer than finding the first character, which is really what this is about. And this ratio will quickly increase as the string gets longer.