when i left they asked my friend to start working weekends, so maybe

That indicates leadership which is overstressed and has a poor understanding of the nature of software engineering productivity.

So this theory could make sense. Some practical advice: don't take it too hard on yourself. If someone says you're slow without backing this claim with data and/or comparisons, you're slow compared to what? Taking into account the startup context, I'd be hardly surprised if they were just trying to make you overwork.