When you find step-thru frames, they are usually marketed as "women's" bikes. Whether the geometry is actually different or not, I don't know. But Trek has some step-thru FX models, and Cannondale has some step-thru Quick models. FX and Quick model bikes should do what you want. I know a couple of riders with FX's (men's), and they're definitely not slow.
https://www.trekbikes.com/us/en_US/b...olorCode=black &
https://www.cannondale.com/en-us/bik...ku=c31351u10lg To be fair, guess I should have mentioned Specialized makes a step-thru version of the Sirrus (marketed as a woman's bike). Again-whether the geometry is diff., or if just narrower bars or diff seat, I didn't look into, but guess a rider would need to try to see if the fit is good anyway. I know I want to ride a bike before buying, others may feel differently.