Is my iPad old ?

If possible try to update the iOS to the maximal possible version

Chances are that the app is optimized for 9.3.1 and your profile indicates 9.1

You have enough space - so that should not be a constraint

As regards Safari - using app switcher - close Safari

Settings - Safari - Clear website history and data

Force restart the iDevice

Hold the sleep/wake and home buttons down together till you see  logo - then release and allow normal boot up

See if that makes a difference - if not please post back with results and we can try and troubleshoot further

Your iPad will not work as fast as the new ones since the processor is now 4 years old. However, there are many users with older iPads that work just fine. Your issue could be with that particular app. You could try other navigation apps to test. You could also do a restore from backup through iTunes. That will install a new copy of the current iOS in case there are any issues with the one you have. Restores are normally recommended to users who are having problems but you could try it and see if it improves your iPad.

Restore: First backup your device using iTunes. Then import your photos to your computer and copy all important data.

https://support.apple.com/en-us/HT201252

That is actually an interesting conundrum - by all means try to restore from backup

I actually advise people the other way around

Backup to iTunes

Then restore as NEW from iTunes - see if the problem solves

This way - you are not worried about something in the content or existing iOS that is corrupted and keeps re introducing the same error

If the issue is solved - then restore your backup

Of course this also serves as a diagnostic test - if restoring the backup restores the problem - then it is likely that something in the content of the backup is contributing to the issue

Restore your device from an iCloud or iTunes backup - Apple Support

A restore from backup should retain all settings and passwords. But please note the emphasis on the word "should". While not common, restores from backup have on occasion failed and no one can make any guarantees. It was only a suggestion to possibly improve performance but if you are not comfortable with it I would not do it.

Your iPhone 6 is far faster. Also the version that gets installed on the iPad is likely different than what gets installed on the phone due to app thinning.

As elcpu pointed out - the fundamentals are different - the underlying chipsets and hardware are different - all you know is that it is extremely likely to work on the iPad running 9.3.1

When doing the backup so all data is saved, do it using the iTunes program on a computer (Mac preferred). Set the backup to be stored locally on the internal drive, encrypted with password.

Yep - you got it!

You are very welcome