As we all know..
Well I disagree. Some tables work just fine without such a field. But they a usually small, special use tables so I am just quibbling over details..
I suspect that you are right, but have never run any tests nor have I seen any test reports to confirm it. I doubt that it makes a large difference except maybe for truly large numbers of records, but then I prefer to use auto-entered serial numbers with fields of type number in the first place as a combination of the KISS method plus the fact that I may need to sort on that field at some point and it won't sort correctly if it's a text field.
As to how data is stored internally, you'd have to ask a FileMaker engineer and they won't be allowed to tell you (I think) due to the info being considered proprietary.
Thank you for your post.
If fields are indexed, numeric values will be generally shorter than text values. That is, the indexing space will take up more room with text values than with numeric values. The performance will be minimal since each value is associated with an internal record number to locate the record.
A "five-digit text number" (like a zip code) will be stored as five characters.