For simple "web apps", typically its a poor understanding of SQL that is the bottle neck. Since most web frameworks include an ORM, many developers don't even know much SQL any more and treat the database as a simple 'object' that you can call methods on and iterate through. Case in point: you're building a dash board for a web based store to show details about the 5 most recent orders. But each order references 5 other items (as a simple example). And each item references an SKU# that can come from 5 manufacturers.

So an optimized query set from an ORM will end up querying the DB like this: First it selects the 5 rows from the order table, but since you left out any join clauses, it then runs 5 more selects on each of the rows returned from the order table to pull the related rows from the items table. So we are now up to 6 queries.

But each row of the item table references 5 manufacturers (again, as a simple schema example). So each of the 'item' 'objects' (5 per order) has to then make a query to fetch the sku manufacturer. So that is 25 more select statement queries on top of the original 6. So using the ORM properly and building out the right join clauses, you would have made 1 query. Instead you made 31. Now lets say that DB is not on the same machine, but a different machine on the network. Throw in 2ms - 10ms latency per query and now you are looking at 62 - 310ms of latency to fetch your data. And that is just for 5 'rows' of data. Do the math for a more realistic dataset, 500-1000 rows or so. Now you're looking at minutes of latency for your data to come from the DB.

The worse case I've seen recently was this exact case. To generate an inventory page to show an html table with about 300 rows, the DB was being queried over 300,000 times with a page load of about 7 minutes (and getting longer everyday). I added the proper join clauses to the ORM and one query was issued which would return the result set in about 700ms which put the page load time to about 1 second. A 1 second page load imo is still not very good. But it seemed to impress everyone that with 1 hour of work, a page load went from from 7 minutes to 1 second....

For what ever reason, most people don't understand this anymore. Instead of learning SQL, the line of thought is to upgrade the machine, move to a 'no-sql' DB, introduce some caching layer, and many number of other ideas. But "lets make sure we are querying the DB instead of iterating through it" is never discussed.