The idea that a specific word count defines pacing feels almost backwards. Pacing comes from what happens in those words, not how many there are. Some chapters that fly by are dense with dialogue and action, others are slow, internal monologues that somehow still pull you forward. I read a fantasy novel recently where a 12,000-word chapter of tense political negotiation had me holding my breath, while an 800-word transitional scene later felt like a slog.
What seems to work for a lot of serialized web fiction is shorter chapters, 1,500 to 2,500 words. It creates that 'one more chapter' rhythm, perfect for scrolling. But in a big epic fantasy doorstop, a 7,000-word chapter can build a whole, immersive world you sink into for an hour. The 'best' definition is probably the one you don't notice because you're too busy reading.
My rough guide? If I'm constantly checking the percentage left in a chapter, it's probably too long for its content.