Its not different to qlikview and quite simple: load only the needed records and fields and build an appropriate datamodel. I think this will be helpful: Get started with developing qlik datamodels.
You did not provide many details about your application (why chart expressions were created with IFs, for example), but I would suggest to invest some time into organizing these expressions with more "classic" Set Analysis, using flags, for example.
Check documentation and videos by Michael Tarallo
A proper data model is the key as pointed out by Marcus and Vladimir has some good points too depending on what is being processed.
You can also try this instead of nested IFs.
For example this:
if(wildmatch(PRODNAME,'*Sony*'),'Sony', if(wildmatch(PRODNAME,'*Toshiba*'),'Toshiba', if(wildmatch(PRODNAME,'*Samsung*'),'Samsung','Other')))
IF(wildmatch(PRODNAME,'*Sony*','*Toshiba*','*Samsung*')>0, PICK(wildmatch(PRODNAME,'*Sony*','*Toshiba*','*Samsung*'),'Sony','Toshiba','Samsung'), 'Other')
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