Server performance tuning bottlenecks

Hey everyone, I'm running a SaaS app (a project management tool) that's seen some decent growth recently. It's awesome, but we're starting to hit some serious performance bottlenecks, especially during peak usage times.

Our API response times are getting sluggish, database queries are taking forever, and server CPU/RAM usage spikes dramatically. I've been trying various server optimization techniques, but it feels like I'm patching symptoms rather than solving the root cause. Users are starting to complain about lag, and it's impacting retention.

So far, I've implemented basic caching (Redis for common queries), added indexes to our PostgreSQL database, optimized some Nginx configurations, and even upgraded our server specs a couple of times. I've also been reviewing our application logs for slow endpoints.

Here's a snippet of what I'm seeing in our logs:

[2023-10-27 14:35:01] production.ERROR: Slow Query Detected: SELECT * FROM tasks WHERE project_id = 12345 AND status = 'open' ORDER BY created_at DESC; -- Duration: 850ms
[2023-10-27 14:35:05] production.INFO: High CPU Usage Alert: server-01 (88%) for 5 minutes.
[2023-10-27 14:35:08] production.WARNING: API Endpoint '/api/v1/projects/{id}/tasks' took 1.2s to respond.

I'm looking for advice on more advanced performance tuning strategies. Are there specific tools for identifying bottlenecks beyond basic logging? Should I be looking into specific database optimization techniques for PostgreSQL or maybe exploring horizontal scaling more aggressively? Any tips on optimizing application code for better server resource utilization would be amazing too.

It's getting critical to fix this before it impacts us further. Help a brother out please...