Probably back when tpt was version <something really small> the save format was still PSv, which was just a bunch of bytes put together in a really complicated way that was highly dependent on version numbers in a bunch of if statements. If the max temperature was increased, then it probably wouldn't fit in the 1 or 2 bytes it was given. I think it originally was much smaller than 10000 (not sure exactly what), and it was increased for the PSv saving format.

Even today, if we increased the limit, temperatures would load less accurate and / or need another byte in the save format.

Temperature has nothing to do with lag, increasing the limit wouldn't cause any noticeable lag at all. Also heat simulation doesn't really take much time at all either.

Size isn't an issue, the balance the game has had and been designed around for years is. Some reactions are designed around 10000 being the max temperature (fusion), some just expect a limit to stop them (NEUT explosions), and all melting temperatures except for a few are under like 2000C anyway. Having things get even hotter doesn't make much sense, since nothing interesting happens at those hot temperatures anyway.