All of the apps I've used are kinda slow at times, but I've had the best experience with the DraftKings app. I'm on Android.
I use DraftKings and FanDuel every day and find those to be the most reliable. I use BetMGM a couple times a week and Caesars roughly once a week. theScore has a nice app, but I only used it a few times. BetRivers looks like it was made 20 years ago, but might be the fastest one without much lag.
We can both get $100 if you deposit that amount. Full details here:
https://sportsbook.draftkings.com/r/sb/huntTJ/US-IA-SB
They're also running this promo for new sign ups. Bet $5+ and Win $200 on NFL, NBA or College Football!