There's an area of front-end performance that hasn't even been touched on here, and it's probably the aspect that us developers are least likely to ever encounter.
The main problem with the front end is that we don't know what devices or browsers are being used, but it's almost a guarantee that those devices are considerably underpowered compared to what we're using.
For example, consider your daily work machine. It's pretty capable of running IDEs, virtual machines, servers, etc, all without complaint. The average users PC has probably a 10th of that power. What might only be a 10ms difference on your machine will be much more noticeable on theirs.
Also, mobile phones are often going to be very old, and probably out of date. Not every phone is old, some were just not an amazing spec to begin with. Considering that most users are browsing via phones and not laptops or desktops, it's important to consider performance there too.
I'm not saying that we should look to optimise to the same degree as code running on the old mainframes or embedded chips, but that we should be at the least mindful of performance. Whilst our code should always be as readable as possible, we should avoid the obvious issues (like nested loops as you've highlight).
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There's an area of front-end performance that hasn't even been touched on here, and it's probably the aspect that us developers are least likely to ever encounter.
The main problem with the front end is that we don't know what devices or browsers are being used, but it's almost a guarantee that those devices are considerably underpowered compared to what we're using.
For example, consider your daily work machine. It's pretty capable of running IDEs, virtual machines, servers, etc, all without complaint. The average users PC has probably a 10th of that power. What might only be a 10ms difference on your machine will be much more noticeable on theirs.
Also, mobile phones are often going to be very old, and probably out of date. Not every phone is old, some were just not an amazing spec to begin with. Considering that most users are browsing via phones and not laptops or desktops, it's important to consider performance there too.
I'm not saying that we should look to optimise to the same degree as code running on the old mainframes or embedded chips, but that we should be at the least mindful of performance. Whilst our code should always be as readable as possible, we should avoid the obvious issues (like nested loops as you've highlight).