Answer Description

Most computers only have one moving/mechanical part - the hard disk drive. A clicking noise is a common indicator of a failing drive. The slowness is likely caused by disk read errors, leaving the logical answer to be the HDD. It is not likely to be a SSD issue because solid state drives do not have moving parts and would not make a clicking noise.

Wikipedia

Click of death is a term that had become common in the late 1990s referring to the clicking sound in disk storage systems that signals a disk drive has failed, often catastrophically.The clicking sound itself arises from the unexpected movement of the disk's read/write actuator. At startup, and during use, the disk head must move correctly and be able to confirm that it is correctly tracking data on the disk. If the head fails to move as expected or upon moving cannot track the disk surface correctly, the disk controller may attempt to recover from the error by returning the head to its home position and then retrying, at times causing an audible "click". In some devices, the process automatically retries, causing a repeated or rhythmic clicking sound, sometimes accompanied by the whirring sound of the drive plate spinning.