You can set the field to "Relational: One value per Album Artist (Auto)".
The way MC works, it will still want to calculate this value for each and every file even though the result is the same for the same artist, which kind of defeats the purpose of a Relational field.
This expression is likely much faster, but it will only refresh the counts if you restart MC:
if(isempty(load(_acount_{[Album Artist /(Auto/)]})),save(GroupCountQuery(Media Sub Type;Album Artist /(Auto/), Artist - Album /(Year/)), _acount_{[Album Artist /(Auto/)]}),load(_acount_{[Album Artist /(Auto/)]})) Movies