When incremental .DAT files are required to update DATs on a client, the amount of processing required to decompress, decrypt, combine, and apply them can increase. Although using incremental DATs can help to reduce network bandwidth, merging the files on the local computer and applying them consumes additional CPU resources during the update. If CPU resources are available, McScript_inuse.exe will consume the amount needed to complete the update as quickly as possible, and release it upon completion of the update. If required, McScript_inuse.exe will yield the CPU resources to a higher priority process during the update. High CPU during the use of McScript_inuse.exe is more noticeable on systems with slower processors and less RAM. It also depends on the number of incremental files required during a given update.
If your customer is complaining about poor performance during ODS - what's your policy/regulation for ODS (during business hours or after business hours, make sure that you have the proper OAS/ODS exclusions in place and perhaps take a look at McAfee Profiler (profiler should help dial in exclusions for OAS/ODS)