Go under the Apple to "about this Mac" and tell us which processor you have, What RAM configuration you have and what storage you have, please.
iMac Intel 27" EMC 2639 CPU Replacement
You have the slowest of the three available i5 processors. You can go to a higher speed i5 or go to an i7.
See this link to see about the Core i7 3.6 GHz CPU:
http://www.everymac.com/systems/apple/im...
Processor Speed: 3.4 GHz Processor Type: Core i5 (I5-4670)
Details: This model is powered by a 22 nm, 64-bit "Fourth Generation" Intel Mobile Core i5 "Haswell" (4670) processor with quad cores (four independent processor cores on a single silicon chip, each with one thread). Each core has a dedicated 256k level 2 cache and all processor cores share a single 6 MB level 3 cache. It also supports Turbo Boost 2.0 (up to 3.8 GHz) -- which "automatically boosts the processor speed based on workload" (so if an application is only using one of the four cores it will automatically increase the speed of the core in use and turn off the unused cores).
iMac Intel 27" EMC 2639 RAM Replacement
RAM Type: PC3-12800 DDR3 Min. RAM Speed: 1600 MHz
Details: Supports 1600 MHz PC3-12800 DDR3 SDRAM SO-DIMMs (204-pin).
Standard RAM: 8 GB Maximum RAM: 32 GB
iMac Intel 27" EMC 2639 Blade SSD Replacement
SSD Blade upgrade
https://eshop.macsales.com/item/OWC/SSDI...
iMac Intel 27" EMC 2639 Hard Drive Replacement
Details: *This model has a Serial ATA (6 Gb/s) connector for a 3.5" hard drive as well as a proprietary PCIe connector for an SSD. If the SSD is not installed at the time of purchase, the connector still is present and later professional or self-installation is possible.
At the time of purchase, Apple offered a 3 TB 7200 RPM hard drive for an extra US$150, a 1 TB or 3 TB "Fusion Drive" (which combines a 128 GB SSD and a 1 TB or 3 TB hard drive) for an extra US$200 or US$350, respectively, and a 256 GB, 512 GB, or 1 TB SSD for an extra US$200, US$500, or US$1000, respectively.
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I think you're between a rock and a hard place on this one ;-{
While you could upgrade the system it will likely cost you more than selling yours so you can get a more powerful model (new or used). Don't forget the risks here of having to do all of the upgrading.
by Dan