First off: why are you adding more than 1500 objects to a rule?
Or more specific: what precise steps are you taking?
Are you adding these objects directly to a rule, a group, or?
Based on that, perhaps we can suggest a way of working that would make more sense if you really don't want to upgrade.
Note that I can't say for sure that upgrading will solve the issue.
However, given that R80.10 is almost end of support, it's highly recommended.
There is zero reason to upgrade to R80.20/R80.30 at this point, go to R80.40 (the current widely recommended release).
There are significant improvements and fixes between R80.10 and R80.40.
If the issue still persists in R80.40, the odds of receiving a bugfix for it are much higher given where R80.10 is in its lifecycle.