Large, but empty tables

I have a problem with the system I'm using: Why does it take many minutes to figure that a table has no or few records in it? This is a huge file that has all or most of its records deleted (so the size of .dat and .idx files reflect the maximum size of the table). Running bcheck -s seems to improve the speed somewhat, but am I expecting too much for near instantaneous response for the first record from the table? First of all, almost all access to the tables is through C-ISAM 3.10 on a DEC 5810. There may be more than 20,000 records in a table. Records are constantly added and updated. At some point, the records expire and they are deleted from the table. Why do the index files get screwed up? Bcheck -s will often report errors and recreate indexes, but why and how would C-ISAM damage them? I get the impression that deleting the files through C-ISAM is what causes the problems. I still don't understand why there is not in Informix a single command to clean-up and resize a table.