If you have a first-generation iPhone 4 (i.e. running the A4 processor) and are considering upgrading to iOS 7 my recommendation would be to hold off until you have a new phone with the processing power to handle it.

After upgrading to iOS 7 after a few weeks ago, I can safely say that while the phone remains usable, it is a lot more sluggish even for basic operations. Sometimes the lag is barely noticeable, sometimes maddenly so.

It wasn’t too hard to get used to the changes in the interface, but I while the UI for iOS 7 is different, I’m not sure it is any better. Learning the new way of doing things felt like climbing a hill to get back to where I was. Except that everything is a bit slower.

So aside from getting to play with the new UI, in my opinion there is no upside to upgrading a first-gen iPhone 4 to iOS 7.

Update (Mar 10): This Ars Technica article takes a more detailed look at running iOS7 on an iPhone 4 and more or less jives with what I’ve experienced myself.

After putting it off for what seems like forever, I recently decided to give in to the nagging iTunes dialog box and upgrade my iPhone 4 to iOS 7.

I have a first-gen iPhone 4, meaning that it runs the older A4 processor. I was already seeing performance issues as I updated my apps to versions designed to take advantage of the newest iPhone hardware Apple had to offer. Everything still worked, but the responsiveness left a lot to be desired.

In this scenario, upgrading to a new OS is probably the last thing one would normally do. Still, hope springs eternal, and after my research failed to turn up any horror stories, I decided that I had little to lose, so I went for it.

Short version: everything is fine.

Here are a few more details for others contemplating the same upgrade:

• If you are running iOS 6.x on a first-gen iPhone 4, you are already used to somewhat sluggish performance. With iOS 7 I find my phone is slightly less responsive, but still very much usable.
• The  changes to the iOS 7 UI have a much bigger impact on the usability of the phone than raw performance. The UI isn’t all that bad and I’m sure that I’ll eventually get used to it. First impressions though are that everything just feels kind of “off” and not what you would expect from using an Apple product.
• Most if not all of the buttons in the UI are gone, replace with text and line-art icons. I seem to have problems ‘clicking’ on these elements, but I can’t tell if that is because the target size has moved or changed, or if the phone just isn’t responding fast enough to my gestures.
• Battery power seems to drain off a bit faster, but it is still too early to say if this is a will become a major problem.

Those are my initial impressions. I’ll update this post if/when I experience any other issues that I think might be of use to others.