I notice this gets asked a lot, and the real friction point isn't just speed—it's about sync. An app can push chapters the second they're live, but if you're juggling multiple apps, you're always chasing. Kindle Unlimited is surprisingly fast for enrolled series, but the selection is its own walled garden. For web novels, the official apps like Webnovel or Dreame are basically tied to their own platforms' upload schedules; they're fast, but you're locked in. I've had better luck with RSS reader apps hooked to author Patreons or SubscribeStar for truly real-time updates, but that's a whole manual setup. The 'fastest' becomes meaningless if you have to check ten different places.
Sometimes the delay isn't the app, it's the translation pipeline. A lot of those 'fast update' claims for translated works just mean they're ripping fan translations quickly, which then get hit with DMCA takedowns, causing bizarre gaps. I'd trade a few hours of delay for stability from an official licensor's app any day. That consistent, predictable schedule from somewhere like J-Novel Club's app ends up feeling faster in practice because you're not constantly refreshing a broken page.