I was watching Halt and Catch Fire and in the first season the engineering team makes a great effort to meet something called the "Doherty Threshold" to keep the responsiveness of the machine so the user doesn't get frustrated and lose interest. I guess that is lost to time!
Old days: Cmd + f, type what you want.
New days: first scroll to the end of the page so that all the contents are actually loaded. Cmd + f, type what you want.
Is just a list of dishes, some with small thumbnails, some without any images at all. If you can't load a page with 30 dishes fast enough, you have a serious problem (you could always lazily load the thumbnails if you want to cheat).
That depends on your scale. If your product is "large enough" it is relatively easy to get into the range of several seconds of response time.
Here are some of the steps you may want to execute before responding a resquest to your user:
- Get all the dishes that have the filters the user selected
- Remove all dishes from restaurants that doen't delivery in the user location
- Remove all dishes from restaurants that aren't open right now
- Get all discount campaigns for the user and apply its effects for every dish
- Reorder the dish list based on the history of the user interactions
Now imagine that for every step in this list you have, at least, a single team of developers. Add some legacy requirements and a little bit of tech debt... That's it, now you have the perfect stage for a request that takes 5-10 seconds.
I mean maybe your DB is a single node running on a potato and your load's very high but you're also somehow never hitting cache, but otherwise... no, there's no good reason for that to be slow.
[EDIT] Your last paragraph is the reason, though: it's made extremely poorly. That'll do it.
It's extremely fast. Super duper fast. And a quick look at the network debugging tab shows why: it loads the shop's entire catalog data (about 3 megs) upfront, and the entire application runs locally with not a single request until you buy something. Now that's efficiency.
Really. Go to their website, clock on KATALOG and click some random buttons, pick a product at random, add it to your cart, remove it from your cart.
The product images are the only things that aren't pre-loaded.