Whenever a running process loads an executable module (typically EXE or DLL) for which 0patch Agent has a micropatch, it needs to load this module from the disk to calculate its cryptographic hash. This operation is usually instantaneous, especially since said file is typically still in the disk cache as it has just been loaded by the process. However, very slow computers can experience further slowdown during system startup when many processes are getting launched (and therefore many modules loaded) simultaneously.
0patch Agent includes a component called 0patch Scanner, which occasionally (after installation, and after new micropatches have been downloaded), scans local disks for new "patchable modules" (i.e., executable modules for which micropatches exist). This operation works extremely fast on NTFS systems but is significantly slower on FAT systems. Note that 0patch Scanner can be disabled if needed.