I would first off clean it up a bit with a free program called onyx which does a multitude of cleaning tasks (caches, junk files etc) and verification of hardware including drives.

I know this is a late 2012 iMac but maxing out your RAM would be my first port of call. Depending on the exact model they did not make some models easy for DIY upgrades of RAM.

If your model is easy to RAM upgrade (DIY I mean) then have you considered just using an external drive?

2012 has TB2 port right? , have you though about simply slaving a USB-C SSD drive off this.... ( via a TB2-TB3 adapter)

If this is one of the models that don't have an access hatch at the back then the screen needs to be removed to do anything to it and that's not a fun or easy job. If you are going to replace the drive then you might as well put something of reasonable size in it, I wouldn't go any less than 256gb

To answer the original question,  Service Plus, Oobe, and PB Tech can all do this as they're AASPs .  

gehenna:

To answer the original question,  Service Plus, Oobe, and PB Tech can all do this as they're AASPs .  

They say why?

Send it to us in Whangarei. :)

What size is it?

The 21.5" models use a 2.5" notebook size drive while the 27" use a desktop 3.5" drive.

Whether you upgrade the internal HD or not I would still max out your memory. OS's and App's are becoming more & more memory hogs and the current High Sierra is no exception.

If you replace your HD with an SSD then the next OS, Mojave v10.14.0, will love you as it is optimised for SSD especially the new Apple File System (APFS).

gehenna:

They say why?

 

I had a similar issue with speed and ended up just getting a Samsung T3 500GB drive for around $160US when Amazon had a deal.

Using Blackmagic my write is around 350MB/s read 425MB/s - so it's plenty fast.

It does take slightly longer to cold boot as it seems to search the internal drive for a system before looking at the TB ports.

I just use the internal drive as extra storage.

Edit - I see the T5 is only $128USD for the 500GB at the moment. That's a great price.

Shapenz:

I had a similar issue with speed and ended up just getting a Samsung T3 500GB drive for around $160US when Amazon had a deal.

Using Blackmagic my write is around 350MB/s read 425MB/s - so it's plenty fast.

 

It does take slightly longer to cold boot as it seems to search the internal drive for a system before looking at the TB ports.

I just use the internal drive as extra storage.

 

Edit - I see the T5 is only $128USD for the 500GB at the moment. That's a great price.