For the most part, your assumption is true, with the exception of SQLTags Historian. It uses 8-byte integers to store timestamps rather than the database’s date/time column type, which assures millisecond resolution. But for other features, like the Transaction Groups, we are at the mercy of the database’s timestamp resolution.
Performance is a fairly complex question, so I’m going to paint with a broad brush here. I’d say that on Windows, MS SQL Server tends to out-perform MySQL, but only by a slim margin. Oracle also offers very good performance. MySQL tends to perform better on Linux.
We do not recommend using PostgreSQL unless you are already extremely familiar with PostgreSQL tuning. We’ve had some bad experiences with PostgreSQL. That said, I know of a few customers who are PostgreSQL experts, and they’re having great results.
All-in-all, much of the performance questions tend to be most greatly influenced not by choice of database, but by schema/table design and query tuning. As in, its not the tool, but how you use it, that is most important.