External drives if connected via USB 2.0 will only run at about 25 Meg
per second. This compares with 60-80 Megabytes for an IDE/SCSI or
SATA internal drive. An external SCSI or SATA can also run at speeds
similar to internally mounted drives.
A slow PC can be caused by all kinds of issues. Malware or a Dirty
drive ( inconsistent state ) will make things run slow. Most likely the
internal drive is highly fragmented. This can cause performance loss over
time. Because it increases with use, most users do not notice how much
the PC is slowing down.
It sounds to me like your system just needs a cleaning and tune-up.
*Cleaning not physical cleanup, but that also should be done a couple
of times a year. ( Remove dust and accumulated gunk on fan blades )