iOS Mail app freezes when opening an email, just the spinning wheel shows

Duckworp Said:

"On my iPad which is currently frozen on Mail, I have used 130 of 256 GB. So plenty of room. Surely it can’t be this reason?"

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Clear your Caches:

See my User Tip: How to Clear "Other" in your iPhone's Storage - Use Backup - User Tip. Caches would be being used for remembrance of application use. It is essentially the same for an iPad. You'd backup your iPad, and then restore your iPad from the backup you've just created. All System Data will delete.

Duckworp Said:

"iOS Mail app freezes when opening an email, just the spinning wheel shows: [...]After a few minutes all works well again before it freezes once again. This happens once every 10 minutes or so and the freeze lasts for a few minutes. It happens on both my iPhone and iPad.[...]"

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Clear Some Space...

System services may be being swollen up. So, try clearing space.

Troubleshooting iPhone Storage:

Internal storage cannot be upgraded. It is what it is. So, the maximum internal storage capacity never increases, even if you upgraded your iCloud plan. Learn More here: What's the Difference Between Device Storage and iCloud Storage? Apple Support

So, what to do...

Clear Space Using my User Tips:

• How to Clear Space from your Mobile Device, and What to Clear: - User Tip. You may have too much installed, and need to clear space. So, see my user tip: 
• How to Clear "Other" in your iPhone's Storage - Use Backup - User Tip. Caches would be being used for remembrance of application use. It is essentially the same for an iPad. You'd backup your iPhone, and then restore your iPhone from the backup you've just created. All System Data will delete.