Try running the benchmark (Help > Benchmark...) and posting the results here.
Model Name: iMac
Model Identifier: iMac15,1
Processor Name: Intel Core i7
Processor Speed: 4 GHz
Number of Processors: 1
Total Number of Cores: 4
L2 Cache (per Core): 256 KB
L3 Cache: 8 MB
Running 'Math' benchmark...
Single-threaded integer math... 3.466 seconds
Single-threaded floating point math... 2.132 seconds
Multi-threaded integer math... 1.003 seconds
Multi-threaded mixed math... 0.631 seconds
Score: 2627
Running 'Image' benchmark...
Image creation / destruction... 1.250 seconds
Flood filling... 0.357 seconds
Direct copying... 0.528 seconds
Small renders... 0.839 seconds
Bilinear rendering... 0.601 seconds
Bicubic rendering... 0.329 seconds
Score: 5637
Running 'Database' benchmark...
Create database... 0.193 seconds
Populate database... 1.088 seconds
Save database... 0.323 seconds
Reload database... 0.094 seconds
Search database... 0.896 seconds
Sort database... 0.681 seconds
Group database... 0.634 seconds
Score: 5499
JRMark (version 25.0.34 64 bit): 4588