SSD as HDD Cache

My friend has the same HDD that I do, a Seagate Enterprise V4 which has a sustained data transfer rate of 175MB/s and ~8000 IOPS and a 128MB buffer. This particular HDD is for servers and commercial 24/7 data transfers. I confirmed it through specs as well, the HDD is almost on par with a WD 10K VelociRaptor which averages ~216MB/s.

The SSD is very low end but I was wondering if it was worth more than trash. He can either shoot for an 850 EVO for $80 which has 500+MB/s and 90,000 IOPS or stick with this. Either way, it's for booting and common applications. Having bleeding edge performance might not be seen but on the flip side we are talking about 2.5x faster performance overall. Not sure if it's worth $80 though

I mean, the difference between 220MB/s and 550MB/s is what...15 seconds OS boot instead of 30 seconds? My old laptop has a hybrid that boots in 29 seconds so it sounds about right for an old SSD being used as a cache.

Will the SSD be able to do more than one thing at a a time? (that's really what makes an SSD so much faster). I'm not sure if that will happen if the SSD is always waiting on the HDD. If the HDD can do one thing at a time I don't see how the SSD could do more than 2 if it's waiting on the HDD. My guess is it'd be trapped at 200MB/s or so.

The boost in speed from the HDD should be around 45MB/s faster than the HDD alone but then again I dunno how the measuring of MB/s even factors in when the SSD is stuck waiting for the HDD. Maybe it'll show no improvement since the SSD is ancient

banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana banana

I was thinking of that myself. 220MB/s read and 100MB/s write is very slow for an SSD. I can get 2.5x that for my buddy and I think he'll appreciate it