Following up on Bob's excellent advice, we can consider what is slow with the current OS.
1) Is the computer slow to boot, and do apps take a long time to launch but run OK once running?
2) Do apps and web functions "stall" for a few seconds in the middle of a task?
3) Do apps work without stalling but seem too slow in executing anything OTHER THAN a save?
The fastest and most effective way for us to advise you on a possible upgrade path is for your to post a system configuration snapshot that will let us diagnose the issue in one go, and without making you play a boring and protracted game of "20 Questions." Fortunately there is a safe, secure way to do that.
We can quickly and within the confines of these forums help you determine what issues are at play if you use EtreCheck, available free from the Mac App Store here:
https://itunes.apple.com/us/app/etrecheck/id1423715984?mt=12
We can see hard data about drive performance, software issues, and RAM usage. Etrecheck is the development of a long-serving and trusted contributor here expressly for displaying information in these forums to help us help you. It will not reveal any personal or secure information.
Run it, select “Report" from the left-hand pane (scroll down to the bottom of that pane to find):
When its report displays, click the "Share Report" icon from EtreCheck’s toolbar and then "Copy report” from the resulting dropdown.
⚠️ Please DO NOT highlight the text in the report before using Etrecheck’s “Copy report” command—that will garble the formatting and make the report slower and harder to evaluate.
NOTE: Changes in late 2018 to the forum software require you use the “Additional Text" icon (see example below) to embed the report into a post:
Paste the report into the resulting “Additional Text” window:
