I cannot pin-point the reason for this strange behaviour but by now I know that it has to do with the expressions (calculations) in my Library Views.
Here is an example of the type of a formula I used to generate the custom tag .Album based my file and folder naming convention:
If(IsEqual([.GenreHL],Classical),Regex(FileFolder(),/#(.+)_(.+)_(.*)#/,-1)[R1]_[R2]_[R3],If(Isempty(Regex(FileFolder(),/#(.+) - (.+)#/,-1)[R2]),FileFolder(),Regex(FileFolder(),/#(.+) - (.+)#/,-1)[R1]))
Due to the size of my library (almost 5TB), doing viewing and searching with such on-the-fly calculations appeared to be too much for the program and/or my system (iMac + NAS).
So I started from scratch as suggested, and minimised the on-the-fly calculations. The heavy workload is now when I populate the fields with values rather than when viewing and searching. The latter is now done based on fixed rather than calculated field values and everything is much faster now!
It is still a bit strange to me that MC23 was much faster with the same complex library than MC24, but I am happy now to work in MC24 in the new manner.
Thanks for your suggestions, which pointed me in the right direction!