I observed such behaviour already multiple times but I never tried to detect the reasons nor to solve or bypass it (it wasn't bad enough to go this way). This means I don't know the real causes behind the issue. But I think I could exclude a few things and limit it to other things.
IMO not related is the RAM + CPU consumption of the machine and QV as instance respectively to the application - unless it's near or hitting the 100%. To exclude any temporary issues with corrupted services, unloaded libraries, disturbed I/O between QV / OS / graphic driver and so on - you should restart the machine.
More relevant might be the fact that each QV object has an own RAM limitation and the specific RAM consumption has an impact on the caching-behaviour which is usually seen by appropriate error-messages or calculation-timeouts or a out-greyed object.
Probably even more related is the behaviour of the syntax-interpreter which tries to detect if there is a known field-reference or a function() as well as references to variables, other expressions and groups and if they are right spelled and syntactically correct - and (assumingly) if they are already cached. I believe that this isn't a simple one-level processing else a multi-level processing between the string-parser and interpreting them as well as checking the correctness and the exists and so on - which jumps forwards and backwards. Further I'm not sure if certain reference-types like variables are re-calculated by typing them.
By a large number of them - in the expression as well as within the parent-objects and the parent-container (container and sheets) and the document - it could mean a lot of processing which couldn't be performed in real-time. Especially if any caching-limits are reached.
Deducing from the above I suggest you tries your expressions within a new text-box, then a new sheet with a new chart and starting of re-creating it from the scratch. What happens? Any changes within the response-times by start and then after which working-steps?
Beside this you could review your UI if the number of variables and expressions could be reduced and/or unified and of course be optimized in regard to the content and performance.
