Server performance tuning woes!

Hey everyone, hope you're all having a less stressed day than my server is currently providing me! My SaaS app, which is a project management tool for small teams, has been getting some really fantastic traction lately, which is obviously brilliant news for growth. However, it seems my trusty VPS is starting to "complain" quite a bit, throwing a digital tantrum every now and then. There's honestly nothing quite like the pit in your stomach when a user messages you about slow loading times or, even worse, a dreaded 500 error. It feels like every time we hit a new user milestone, the server decides it's time for a coffee break, right when it should be working its hardest. The specific issues are becoming a real headache: page loads are noticeably slower than they should be, we're getting occasional 500 errors during peak usage times, and database queries are taking what feels like an eternity to resolve. I've also noticed CPU usage spiking unpredictably, sometimes hitting 100% for brief periods, which is certainly not ideal for user experience. It's a fairly standard LAMP/LEMP stack (I've tinkered with both) running on a decent VPS, but it just doesn't seem to be coping with the increased load gracefully. Naturally, I haven't just sat back and watched the chaos unfold. My initial attempts to fix things involved the usual suspects: I upgraded the VPS plan a couple of times, thinking more RAM and CPU would magically solve everything (spoiler: it provided temporary relief but didn't stick). I also implemented basic Redis caching for some of our more common database queries, which helped a little, and put Cloudflare CDN in front of everything to offload static assets. I've even spent some quality time doing basic SQL query optimization, adding indexes where they made sense and refactoring a few particularly heavy queries. While these steps offered some breathing room, they honestly felt like putting a band-aid on a gushing wound; they didn't really get to the core of the problem and certainly haven't provided the long-term stability I need. So, here's the burning question for the collective wisdom of this community: what are the next steps for serious web server optimization and performance tuning beyond these basic fixes? What practical, real-world strategies have you guys found effective for a growing SaaS application that absolutely needs to scale reliably? I'm talking about the nitty-gritty stuff – advice on Apache or Nginx configuration tweaks, deep-dive database tuning for high concurrency (we're using MySQL/MariaDB), or even OS-level tweaks and kernel parameters that can make a difference. I'm ready to roll up my sleeves and dive into more advanced techniques to stop this server from throwing a tantrum every time a new user signs up. Any insights or expert advice would be incredibly welcome. I'm really eager to learn from your experiences and get this server performing like a well-oiled machine again, rather than a grumpy old man constantly complaining about its workload. Thanks in advance for any guidance you can offer!