I’m not sure if you would consider it a custom solution, but perhaps you could use LOAD DATA INFILE which is a very fast method of importing CSV files, if it’s suitable.
I was finding importing 10-20k records very slow and had to fiddle with PHP setting to prevent time-outs etc. I switched to LOAD DATA INFILE and thought it hadn’t worked - the timer I had included in the SA file returned zero seconds. In fact the import had worked, but the process took less than a second. It’s possible to import 10s or even 100s of thousands of records per second with this method.
I created a custom PHP file to do this and called it within a SA as an API. However, this was quite a while ago, before custom queries were available in Wappler. When custom queries appeared, I thought I would replace my PHP file. This worked, but I had to modify a Wappler file, with @patrick’s help, but only using some ‘incorrect’ syntax. I’m not quite sure what the issue was, but in any case, I decided to stick with my original solution. It might be that it would work now, simply using a custom query. There have been quite a few changes in the relevant Wappler file since I raised the issue here.