How to Identify Cause of Calculating Threads?

I have a light excel file around 10mb. However, it takes a while to update / refresh with any formula given that it always shows Calculating threads (sometimes it's 6 threads, other times 14 threads)

There are quite a number of worksheets - maybe around 10. formulas refer to other worksheets in the file and there are formulas linked to 10 external files (mostly VLOOKUP, XLOOKUP).

Is there a quite way to identify what's causing the slowdown / calculating threads so I can address? by theory, what causes this?

I'm tired of waiting 10mins just to get a simple sum or division. It seems like the Excel needs to recalculate all formulas every single time. Already tried making calculations manual instead of automatic. It helps not to update every single time, so I can wait for all formula adjustments before clicking F9. But, another 5-10mins wait once I do update.