One thing to know about transactions is that they track data and then write it. It’s not the opening that slows it down. I have a question though, what is your source data? Do you have a big CSV for something? Can you do a db to db transfer instead? There’s another tool called the BCP utility.
Edit: SQL server/ssms have tools for doing migrations and batch imports
It’s how CPUs do floating point calculations. It’s not just javascript. Long story short, a float is stored in the format of one bit for the +/-, some bits for a base value (mantissa), and some bits for the exponent. As a result, some numbers aren’t quite representable exactly.