Mac OS Monterey 2015 27" monitor hook up?? OR New Mac? But which one?

Running a graphics-oriented business on a 2015 iMac? I guess it is possible but you will constantly be struggling with advanced software (e.g. Adobe Photoshop, Indesign ...) that really needs a modern platform, and you won't be able to update that software on such an old system. You will also be facing the 2015 era hardware wearing out, such as the slowing down you noticed which could be due to many things, including the internal drive wearing out (but it could be due to other things as well).

Your Mac can go to 32 GB RAM but I believe the limit on its speed and utility will be more from the internal drive. Best would be an SSD, using an HDD would be almost unusable and a Fusion drive is somewhere in between. Note that Adobe software writes prodigious amounts of data to the internal drive constantly for "scratch" or temporary storage so the speed of that drive is important, as is free space. Lightroom can use up 150 GB internal storage (temporary use) in a blink of an eye.

You are using software that costs in the thousands of $, but your Mac is an obsolete and very slow model. New Macs cost as little as $999 but you need one with enough horsepower. Apple and companies like OWC are reputable sellers of used Macs that come with warranties and service contracts, those will be more affordable. There is an Apple Authorized Service Provider (AASP -- third party certified Apple service center) in my neighborhood that sells used Macs in excellent condition and guarantees them through an extended warranty. I have seen some top of the line systems available through that store at relatively low prices. You might look into those options to replace your Mac if a new Mac isn't in the budget right now.

No you cannot use the old iMac as an external display, this functionality has been dead since 2014. Please carefully read Use your iMac as a display with target display mode - Apple Support. The wisest thing you can do if you need a display is to purchase one, I'd suggest looking at Samsung's 4K display, you find them on Amazon for $399, that display would make an excellent addition to a Mac Mini or. your Lenovo.

Luna display is not a good option if you have ANY intention of doing any type of graphics, read the fine print on their website!!!

Also, please remember the 2015 is now quite obsolete (10 years old!!) and cannot run current versions of Mac OS, possibly has the original HD or Fusion drive which could fail at any time. If I were in your shoes I would recommend investing in a new Mac Mini which will far out-perform your iMac and future proof you for the next 5-10 years rather than investing anything on a 10 year old iMac.

I'm in total agreement with steve626 in that you're trying to run a business on a "Vintage" (Apple's term - next step "Obsolete") iMac that's 9 years old. IMO you'd be best served by getting the top of the line Mac Mini (10-Core CPU 16-Core GPU)

with 32G of RAM and a 1 TB SSD disk. Get the top of the line as it has more Thunderbird ports and Ethernet for faster connection to the internet.

Then pair it with a 32" LG 32UN500-W Monitor 32" UltraFine (3840 x 2160) Display, AMD FreeSync, DCI-P3 90% Color Gamut, HDR10, Built-in Speakers for ~$330 @ Amazon (you can never have enough screen real estate). There are also 27" monitors anywhere from $100 to $500 depending on the features.

Note: all new Macs are no longer upgradeable by the users or Apple. The RAM is integrated into the motherboard and the drive is also. So order what you need up front.

Old Toad wrote:

Get the top of the line as it has more Thunderbird ports and Ethernet for faster connection to the internet.

Thunderbird? Did you mean Thunderbolt?

All of the M2 and M2 Pro Mac minis have Gigabit Ethernet (standard) or 10 Gigabit Ethernet (optional). Note that the optional 10 Gigabit Ethernet ports will not connect to equipment that runs at Fast Ethernet (100 Mbps) speed. Only to equipment that runs at Gigabit Ethernet speed or faster.

I looked at the Mac Mini specs on the Apple web page ... that's an impressive computer and quite competitive with the iMac options available. Having the separate display can also be advantageous, if the display fails or you want to upgrade to a better display, that is easy with the Mac Mini, not so easy with an all in one iMac. The higher end Mac Mini models come with 4 Thunderbolt ports, plus separate HDMI, plus ethernet, plus 2 USB (5 Gbps), that's quite a configuration.

Now that's what I call a cool port! 🤭