SMS Thread / General Limit? Is there a limit within Android, or possibly set by OEMs (heard this from a friend who is a technician), of the total amount of SMS messages you can have? As in, messages within a thread  and/or all threads combined? Not just the limit within conversations which you can manually set. I mean a limit android has coded into it, or one that OEMs set, that actually would "limit" (whether by slowing down or stopping) the number of SMS you can have? 

And I'm not talking about a couple thousand, but even in the hundreds of thousands in a thread?
I cannot provide a definitive answer, but some folks have had over 20,000 messages, based on this Reddit discussion. So if there are any limits, it may have been set by the OEM (or their custom app). Using 3rd-party SMS apps might provide a workaround to the limit.

I would bet that if are hundreds of thousands of messages, there could be a significant performance issue, although that may depend on how the app processes the records. Go SMS Pro, for example, does not load the messages all at once. But if it reaches the point where it has loaded already a lot of messages, the app slows down.

UPDATE #1: To paraphrase Izzy, the storage limit will also trigger the "insufficient memory" error, usually when there's less than ~25 MB free space on internal storage. This would also prevent the SMS from being stored (plus the possibility to send some on your own).