Good morning @scandinaviannerd,
1. When I added subdomain (dummy.iqm.no) where I have installed SPAI plugin, I had earlier also added existing domain (iqm.no) where NO spai is installed. How can SPAI show that the free version has optimized 15 images on my existing domain? Especially when this plugin is NOT added on my existing domain?
I am not really sure if I understand your question, but our system sees every optimization from SPAI and SPIO regardless of the credits you are using (free or paid). So, I believe that somehow you used our services with one of our plugins at some point.
2. Despite this plugin is working, GTmetix is showing bad score for my subdomain developing site: https://gtmetrix.com/reports/iqm.no/MpLwMZQ3
The scores are a way to tell you how good a website is performing in general. If there wasn’t a score, it would be too difficult for the average user to understand the results. Such scores are a combination of many factors, most of them unrelated to ShortPixel. Since these factors change with each test, it is possible that they affect the overall score.
The loading time, in general, is better with our plugins. This is what ShortPixel is aiming for, a better image optimization and thus less page size and less loading time. Like we said, looking at the general score can be misleading.
2a. Which caching plugin is works best together with SPAI to improve GTmetix score?
Every website is a different world, therefore we cannot recommend a caching plugin. My suggestion is to try each of them and see which one adapts the best to your site.
I am also getting this issue on gtmetrix:
The following resources are missing a cache validator. Resources that do not specify a cache validator cannot be refreshed efficiently. Specify a Last-Modified or ETag header to enable cache validation for the following resources:
Please have a look at this article: https://help.shortpixel.com/article/227-i-receive-specify-a-cache-validator-when-testing-with-gtmetrix-why
Best,