Is there a reason to not use VPN all the time on iOS?
When you use a VPN to send all your traffic to another network, you are reducing the ability of the internet to route your packets directly from your phone to the servers and instead are forcing the network to put the packets the VPN embeds to travel first to another network and then to the destination.

In the case where you need to hide / encrypt traffic, this downside is clearly worth bearing. For general purpose use, you will almost always add latency and perhaps bandwidth constraints that make the network response slower - even assuming all the encryption is done in hardware and has no effect on battery life on the device nor on performance due to delays in the encryption / decryption.

Also, your network is far less resilient and you have introduced a single point of failure (the VPN termination) into your path to the internet.

These downsides can be mitigated when you apply VPN to some traffic as opposed to all traffic and ensure your VPN is well connected to the internet.

In some cases VPN speeds the network if the traffic you send elsewhere is throttled and the traffic to the VPN is not throttled, but this is an edge case and worth considering, but rarely a reason to use VPN exclusively.