Hi Trilochan,
Few areas you can have a check.
1. Your network bandwidth is very good (2G not recommended, To be on 3G/4G or wifi)
2. Response time of your server is good.
3. Are you connected to any VPN from mobile ?
4. Also provide your architecture.(central hub or embedded)
5. Try to run the same from google chrome from your smartphone rather than fiori client and see the performance.
6. By only Windows and IOS devices, do you mean it works good in android ? (Not sure if you have deployed same client for android too)
Regards,
Tejas