Regarding Synthetic keys/Synthetic Tables, the first recommendation is to read through this post created by Henric Cronström.
QlikView or Qlik Sense may end up using an excessive amount of time and/or memory when the number of composite keys/Synthetic Tables increases and depending on data amounts, table structure and other factors.
Unfortunately, the actual limitations are virtually impossible to predict, which leaves only trial and error as a practical method to determine them.
Best Practices:
If they are UNINTENDED Synthetic keys/Synthetic Tables:
Please eliminate the Synthetic keys/Synthetic Tables as much as possible
If they are INTENDED Synthetic keys/Synthetic Tables:
Please do the performance test and find out the best data model based on the testing result.