Install Redis and Cloudflare that will greatly improve. From the rest is to wait for version 4.4 that will solve the issue of images

I agree, this is what I'm using. IPS seems to have a good amount of framework around Redis which seems to be the preferred caching method.

 

If you've turned on Cache page output for guests , turn it off.

I have tried that yesterday briefly and realized it isn't going to work so switched off after 1-2 hours.

So you think that IE loading site in text format is due that cache page for guests....?

 

I have tried that yesterday briefly and realized it isn't going to work so switched off after 1-2 hours.

So you think that IE loading site in text format is due that cache page for guests....? 

Yes, guest page after refresh time, Chrome browser and IE do not load the exact CSS key. Therefore, CSS does not download to these browsers on a this page.
Clear cache manually on nginx caching temporarily fixes this.
Therefore, it is better not to use this guest page.

My performance is even worse. I try everything but there are no results. Please help.

 

My performance is even worse. I try everything but there are no results. Please help.

The results you’re seeing might be related to your server and network configuration. I’d take a closer look at your setup, and consider signing up for a free (or paid) Cloudflare plan.

I’ve only made minor adjustments to the default theme, and the score on my community is 35 on mobile and 88 on desktop. On the server side, I’ve spent a lot of time tuning PHP, nginx, the database server, and more, and I’m using a paid Cloudflare plan as well.

Switch to Nginx is an option. You're not gzipping the output, you're using a lot of third party code that's blocking page render, as OpenType told you, do you really need all of them? You're loading 1.5 Mb of javascripts.. then this 1.5 mb must be parsed..

 

You can also switch from Google Analytics to Matomo, less intrusive and you're not feeding the evil..