There’s nothing inherently slow about Basic. There were several implementations of Basic compilers that were plenty fast enough. Sure, they weren’t as fast as C, C++, Pascal, and Fortran, but they were good enough to make commercial software with.
Many early BASIC implementations for microcomputers (such as tiny BASIC) prioritized memory usage over speed, so they tended to favor interpreters. But as others have noted, numerous compiled BASIC implementations existed as well.