Generally, yes. Performance will be affected during the backup task (especially the first one) as CCC reads the entire source volume and writes to the destination volume. If your work is "disk bound" — that is your applications are reading or writing to either the source or destination, then you'll notice a performance hit. If you're just reading email or writing a document, then you probably won't notice the performance hit.
What happens if files are modified while they're being copied?
If your source volume is an APFS volume, then CCC will create a read-only snapshot of that volume and use that snapshot as a source for the backup task. With this configuration, any changes that you make to files on the source during the backup task will have no effect on the backup process. Likewise, those changes will not be part of the backup – expect the backup to contain exactly what was on the source at the moment that the backup task started.
If the source volume is not APFS-formatted, then some consideration should be given to the modification of files on the source during the backup task. Typically it's OK to work from the source volume while you're copying it, with the understanding that if CCC copied a file, then you open it, make changes, save it, then CCC completes the backup task, the modified version of your document is not backed up (this time around). Typically that's no big deal, the modifications will get backed up the next time the backup task runs. More importantly, though, if you're working with large files (mounted disk image, Entourage email database, VMWare/Parallels container) during the backup operation, it is possible that those large files could be modified while CCC is backing up that file. This won't affect the source file, but there's a good chance that the backup version of that file will be corrupt. For this reason it is a good idea to stop using applications that may be modifying large files for the duration of the backup task. Again, keep in mind that this is only applicable for non-APFS source volumes.