The slowest part of a computer is the hard drive. When executing an operation like Recover by Tags, 4D Tools reads from one data file and writes to another. Most of the time spent executing a recover by tags is spent reading or writing to the hard drive. There are some ways to improve the speed of accessing the drive but it depends on the types of hard drive(s) you have.
Assuming that you have a computer that has two SCSI drives, the fastest configuration is to read from the existing hard data file on one drive and write to the new data file on the other hard drive. The same will be true if you have two IDE hard drives. If on the other hand you have a computer with one SCSI drive and one IDE drive, it will most likely be faster to work with just the SCSI drive.
The reasoning behind using two hard drives is that you are in fact running the two devices in parallel. That is they are both running at the same time. The reason to go with just the SCSI hard drive when you have both a SCSI hard drive and an IDE hard drive is that the SCSI drive is sufficiently faster than an IDE drive, and using the IDE drive will probably slow the operation down.
Of course these factors are relative. Other facts such as how old the equipment is can also make a big difference.