While I haven’t found an easy workaround the slowness of the database, there are two ways (that I’m aware of) to go faster. Both rely on the fact that you can deal with basic elements in-browser only:
1- Decompose your custom things
Let say you have a custom thing named Customer which is made of a name (text) and age (number). Instead of having a custom states that is of type List of Customers, you would have two custom states: a List of text and a List of numbers. Items from each list are linked by their position index so List of text's item #4 goes with List of number's item #4.
This method allows you to quickly add/delete/edit items without waiting on the database. You can then create/modify things in background (Scheduled custom workflow).
However I found that as soon as things get little complex, workflows and actions can become very messy thus hard to debug and maintain.
2- Parse your data in javascript and use JSON to talk to Bubble
This is the method I prefer; it is way more flexible then the first one. But you need to have some knowledge in javascript despite Bubble being a no-code platform. The post your are reading explains how to use it. Since a JSON string is only a text element, but with a specific syntax, it can be used and passed around in Bubble in-browser only.