Longer or delayed withdrawal is a big issue for me, one thing I hate in online casinos is the delay in withdrawal and redirecting to a website when using an app.
Another annoying thing about online casinos to me while using an app is the pop up ads, these popped up ads take upto 15 seconds at times and that isn't cool for me.
Most times, the online casinos require heavy network usages and might take longer time to load up if not having the required network for operation.
Delay in withdrawing suck and what annoying the more is the fee associated with them. I prefer when a Casino remove fees the way how busy every blockchain is but most of them dont do that, they rather place a static fee for withdrawals especially now that all coins have lower fee, they still feed from users withdrawals.
I think ads cannot be removed from casinos, most of them are usually promotional that are within to make players know what is happening and what they are missing or give notice of an events, they are usually not an outside events, this is a most in casinos and they are just few Ads.
The lighter the games, the faster the games can load quickly, most of these casinos now want to impress players with good graphics thats why we do experienced slow load time.