Went to Best Buy and fought the Black Friday crowds to buy 2 each 120 gigabyte SSD drives by Piny, tiny cute little 2.5 inch laptop drives actually.
SATA 3 speeds, 2 million hours of use life supposedly.  

Black Friday price was $17.99 each

I popped one into my oldest Linux box, which is old enough that it only will support SATA 2 speeds which the little drive also does jest fine, so the little drive did great as my bootable drive and the old Dell machine feels like 3x faster than it used to be and 10 times faster when installing updates, etc.

I velcro'd the little drive into place on top of the existing platter hard drive since it weighs nothing and a proper Dell drive adapter for it would cost as much as the little drive itself did on Black Friday sale.

Linux is EASY compared to Windows, I was done with the upgrade in 10 minutes and had the software all installed in another 20 minutes (most of which was copying bookmarks over and tuning all the setting to be the way I like them).

This was the best bang for the bucks pure speed upgrade I have ever done on a computer.  

EVER.

Yes, drive sizes have certainly gotten huge lately, generally speaking.   Piny has been selling off their older stock, they moved their 80 gig SSD stocks out last month (for 20 something dollars) but 80 gigs of SATA 2 was no real improvement to my eyes.   120 gigs of SATA 3 for less money, oh yes I will go wait through the lines.

SATA by itself is getting to be an old standard now.   Most fast modern machines use M.2 stick drives (and have the slots for them). but all machines have SATA cable connectors and matching cables because that runs all the DVD drives and spinning platter drives, so the use of SATA 3 cable standard is still with us for a while.

SATA 3 drives at 120 gigs (running at 3 gbs back compatible speed) is still a sizable improvement to me.   I ran Linux Mint on an 80 gig mechanical hard drive and all my entire everything can fit inside of 20 gigs of that 80 gig space very easily, except for the Steam Games which a single game can eat up as much room as all the rest of the OS uses simply because games are such HUGE things.

Using a new SATA 3 drive because it is back compatible to all the SATA cables on all the older machines out there makes it a bit of a recommend item that can "drop in" for most of us.  

Make sure your machine has SATA 2 or SATA 3 support on the motherboard before buying SATA drives, of course.   Go back another generation into the wide floopy ribbon cables and you can't use an SSD at all on those machines.

Congratulations ------- you have an Intel processor.

Because you have Intel Inside, you now get to experience all the slowdown joys of mitigating all the various flavors of Intel Predictive Processing Illnesses, something that supposedly sped you up by 5-10% when Intel first did it but is now slowing you down by 25% (or more depending on the task you are doing).

A dodge I read about is folks going back to much older versions of Mint that they kept the DVDs for.  This trick is not without its own built-in flaws as some of your stuff requires the more modern software versions and drivers that are included in the best most current Mint revisions.   Plus, you will sit exposed to the Intel Predictive exploits that already are out in the wild, plus all the new ones being invented weekly.

For example, at Linux Mint 18.3 you are two revisions behind on the current Linux kernel.   You might try getting current as on your Mint Version as Linus and the boys have actually fixed (fixed inside Linux itself) some of the things MS and Intel were writing some very bad clumsy early mitigations for, mitigations that actually made things worse (if you can believe that).

This whole thing is a "dammn if you do, dammn if you don't" situation,  but since we both have Intel based machines we will try to get it all to continue working a decent enough speeds.  

Try getting current with your Linux Mint and see if that fixes anything.

Fair warning .........

If you are still dual booting be aware that Intel and MS will load your machine up at night with "predictive bug fixes" on the Windows side that slow the whole machine down to no end.

Consider getting rid of the dual boot and going with 100% Linux.   Get a brand new current installation DVD and when you install it tell it to let Mint have the whole drive.

If you must stick with Windows, then you are in the same boat as my wife, suffering endless aggravation and ever increasing slowness.

Let's assume the worst, that you have some sort of equipment slowly failing.   Do you have an old hard drive of the same type that is in the machine now that you can use to swap out for testing purposes?

I keep the old drives around, and have stuck one into a sick machine for testing (result was it ran the old OS version at the same speed as it did before, but it indicated the sick drive needed a full Mint install off a DVD and to be allowed to totally re-format the faster sick drive while doing so.

Before doing all this, reboot your internet access hardware and your router and your machine one more time so your entire system has gotten a fresh boot on all items.

There are lots of boxes serially involved in your computing experience after all, and "waiting endlessly" can come from any of them.

Old;   HD shows 532 gb free, not a hd problem.
Apart from e-mail and some minor wordprocessing, I just use a pc for Internet.
When I was working, pc was used as a pc, now just web access.  
Maybe like me, it's just gettin' old !!!

speedtest;  ping 10ms, download 16.07mbps, upload 1.58mbps
the test identified the wrong internet provider?????
Cheers!
ps; the slowdown is intermitent, just when I want it fast!!!!!!!  rats