The disk seems good to me. "Pre-Fail" attributes are those that, if below (or equal to) threshold, may indicate pre-failure (imminent disk failure). "Old age" attributes are the ones that indicate normal wear and tear.
So, a reallocated event count of 200/200 with threshold 000 ought to mean "No reallocated events", i.e., "No errors".
This is what I read on my home unit:
5 Reallocated_Sector_Ct 0x0033 100 100 036 Pre-fail Always - 1 10 Spin_Retry_Count 0x0013 100 100 097 Pre-fail Always - 3 12 Power_Cycle_Count 0x0032 100 100 020 Old_age Always - 105 184 End-to-End_Error 0x0032 100 100 099 Old_age Always - 0 187 Reported_Uncorrect 0x0032 100 100 000 Old_age Always - 0 188 Command_Timeout 0x0032 100 096 000 Old_age Always - 4295098559 197 Current_Pending_Sector 0x0012 100 100 000 Old_age Always - 0 198 Offline_Uncorrectable 0x0010 100 100 000 Old_age Offline - 0 199 UDMA_CRC_Error_Count 0x003e 200 200 000 Old_age Always - 0 As you can see, I got a sluggish command timeout sometime or other, but the other parameters are (touch wood!) read from a healthy disk.
See also here.
Anyway, check out Windows Event Viewer (eventvwr). If the hard disk has problems, even if they are not reported by SMART, you ought to see something in the event log relating to disk errors, or maybe filesystem errors. If you see nothing of the kind, you of course still have some problem - the system didn't slow down by itself! - but they are not disk problems.
For example, once I experienced similar symptoms (only much worse). The hard disk was working... then sometimes it would log "hardware disk errors" that the SMART wasn't seeing at all. Windows signaled "Delayed write failed, data might have been lost" (and now that I come to think of it: did you see that message popping up? If the hard disk is going, you should have). I removed the disk and connected to another computer to run some tests. Everything worked. Sheesh. So I put the disk back. But this time it kept working perfectly. Only then I remembered that reconnecting the disk had felt much harder than disconnecting had been -- just as if the connector was already partially loose. It might be worth it to check.
Else, you might be interested in some tool such as Auslogics' BoostSpeed. It's not perfect (it falls for the .wid btrez.dll 'error' - but it's fixable and there's a workaround) and it falls a bit on the scaremongering side when it reports any registry anomaly as a sign of impending doom, but it does its work, and IMHO it's worth the money.
Just to be sure, you can download an ISO of some antivirus - Kaspersky has a free version, and there are others -, boot from that, and make sure you're not slowed down by some unwanted "guest".
But before doing anything else, however, backup all your valuable data on an external device. That way, whatever it is happening, they ought to be safe.