Is iPhone 7 still compatible these days?

The iPhone 7 cannot update past iOS 15.8. Its the last version it supports.

This should not cause it to stop working or lag in any way, nor does it mean it can't keep up or connect to cellular.

The data speeds would be down to her carrier, not the device itself or it being old. It can connect to the cellular service the same way ti always. It won't connect to newer technologies such as 5G, but it can support the same speed and bandwidth it supported from the beginning.

You can try restarting the iPhone, or restoring it to factory settings and setting it up again, and see if that helps its general performance.

You can also look at models slightly older than an iPhone 15 to upgrade to. It does not have to be the brand new one if you don't want to. Something from 2020 or 2021, would still work better and faster than an iPhone 7.

iPhone 13, iPhone 12 for instance.

You can get a refurbished model from Apple for a reduced price.

click here ➜ Refurbished iPhone Deals - Apple

As far as the latest iOS no the iPhone 7 can’t be updated and isn’t compatible. I wouldn’t suggest buying another one as it will likely run into issues with app compatibility as developers stop supporting some older iOS versions and that could impact any apps she may enjoy using. Definitely don’t have to go for expensive newer models but would just look for something that would have a longer window for support.

Straight Talk has SE for 100, you can purchase it online, or maybe it's available in Walmart stores. God bless your mom.

check it https://www.straighttalk.com/all-phones/iphone-SE-3rd-generation-prepaid