That probably depends on what is causing the slowdown. Is there a lot of CPU usage during that time? My guess is there isn't and the slowdown is due to all the disk accessing to read all the subdirectories, in which case there may not be anything that can be done.
Maybe if Opus ran the counting thread at a low priority (if it doesn't already) it would improve performance of other things, but I'm not sure how much thread priority affects directory reading. The problem's probably going to be that thread sending the disk heads all over the place to read the directories while other programs want the heads somewhere else to read their data (or the swap file).
Some of the disk defrag tools let you optimize directory reading by moving all the directory data together; that might improve things a lot.
There's a bug in the current version of Opus which means that in some cases a separate thread can be launched for each folder when calculating these fields, rather than doing each folder on the same per-Lister background thread. This could be contributing to the slow-down (eg, if you have 7 tabs each with 20 folders in each, you would have 140 threads running simultaneously instead of 7.)
If this is the problem then it will be fixed in the next version.