Investigate cache optimization
Getting the site to use less bandwidth and CPU would be good. One way to do that is caching.
Client-side caching means the web browser would only request resources when it knows its updated. This would save the server having to send data at all.
Server-side caching would mean the server only generates pages using PluXml when it needs to, and serves static pages otherwise. This would save CPU and allow more requests.
As an example, I set up the website in a Debian container on my home server (old gaming machine), and ran ApacheBench telling it to make 1,000,000 requests, 10 at a time to the homepage HTML (no images, no CSS):
This maxed out my CPU and gave me a load average of 14 for 10 minutes. As I have 4 cores, this means Linux was overloaded 350% on average. The results showed that the server could handle around 593 requests per second, with each request taking 16ms to process. 10% of requests took double this time.
Running it again with some basic caching enabled (PluXml says everything can be cached, but this probably isn't the case for some objects such as CAPTCHAs) kept the CPU at 3.5 load during this, indicating the bottleneck was no longer CPU usage. After 86 seconds, all million requests were served, with all but one taking 1ms to serve. The initial request to generate the page took 354ms, which is strange but not a big deal thanks to caching. 11613 requests were delivered per second, around a 19x speedup.
If @Deevad can confirm that OVH can provide mod_cache for the site, it might be possible to optimize PluXml. Most of the work would be removing cookies and signaling to the cache to update the page when there's a new article or comment.