However I hardly ever shutdown the computer, do you think changing to SSD will make difference, I thought hard disks were most used during startup.
a
HDDs are limited by some major issues: 1) maximum sustained read/write speeds around 70-120 MB/s for laptop HDDs [and 200-250 MB/s for the fastest 3.5-inch HDDs] -- fast SATA SSDs can read and write around 550 MB/s [and the fastest PCIe SSDs approach 3,500 MB/s], so they can move data much quicker - if your HDD is an OEM, it is likely around the 70 MB/s range; and, 2) latency - the head on the HDD must physically move to the physical location where information on the disk is stored in order to read it, and may need to move again to write - SSDs do not have to do this, as they do not have moving parts, and have virtually no latency at all.
So, just with normal usage, such as opening programs, using multiple Apps simultaneously (that results in some caching), and creating/moving/exporting files, SSDs make an insanely dramatic difference. The time it takes to open an App once you add a SSD could be reduced to 1/10th of that it currently is now with the HDD.
Further, where as Mac OS X does a good job managing RAM, the last few versions of OS X seem to strongly prefer having SSDs, and I find this to be especially applicable with Sierra.