Slow WordPress plugins are one of the most common reasons for high CPU usage, slow page loads, and even server-side errors. When a plugin runs expensive queries or heavy background tasks, it can overload PHP-FPM or MySQL. In this guide, you’ll learn how to identify slow plugins using both WordPress tools and server-level diagnostics so you can fix performance issues before they become major problems.
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Slow WordPress plugins are behind many of the VPS performance issues people experience — high CPU usage, slow page loads, PHP-FPM spikes, and even 504 Gateway Timeout errors. A single inefficient plugin can overwhelm your entire server, especially on smaller VPS instances.
In this guide, we’ll walk through several reliable ways to identify slow plugins using both WordPress tools and server-level diagnostics on Ubuntu. This approach works on any stack, including Nginx, PHP-FPM, MariaDB, Virtualmin, or a manual LEMP server.
Why Plugins Slow Down a WordPress Site
A slow plugin can cause issues in multiple layers of the server stack:
1. PHP layer
Heavy plugin code
Slow loops
High PHP-FPM worker time
Excessive autoloading
2. Database layer
Large or unindexed queries
Too many meta lookups
WooCommerce product queries
Cron jobs fetching remote data
3. WordPress hooks
Plugins hooked to
init,wp_head, orthe_contentUnoptimized admin-side actions
High-frequency AJAX calls
4. Network layer
Remote API calls
Tracking scripts
External requests with slow response times
Identifying the source helps you decide whether to optimize, replace, or remove the plugin.
Step 1: Use Query Monitor (Best WordPress Plugin for Diagnosis)
Query Monitor is the most powerful WordPress debugging tool available.
Once installed:
Login to WordPress Admin
Visit any slow page
Open the Query Monitor panel
Review these sections:
✔ Slowest database queries
Shows which plugin is generating expensive SELECT queries.
✔ PHP errors & warnings
Unoptimized plugins often trigger warnings.
✔ Scripts & styles
If a plugin loads many CSS/JS files, load time increases.
✔ Hooks & actions
Shows what executes on each request.
Query Monitor highlights each plugin’s impact, making it easy to spot bottlenecks.
Tip: Always test both the frontend and the WordPress admin dashboard — some plugins behave differently in each environment.
Step 2: Use WP-CLI to Profile Plugin Performance
On Ubuntu or any VPS:
List installed plugins:
wp plugin list
Temporarily disable all plugins (diagnostic mode):
wp plugin deactivate --all
Then enable them one by one:
wp plugin activate plugin-name
Reload your site after each activation.
If the site becomes slow after enabling a plugin → you found the culprit.
This is very effective when:
WordPress admin is slow
WooCommerce backend lags
A plugin locks backend operations
Step 3: Use Server Logs to Determine Slow Plugins
When plugins overload PHP-FPM, worker processes slow down or crash.
Check PHP-FPM slow log:
grep -r "slow" /var/log/php*
Or:
tail -n 50 /var/log/php8.1-fpm.log
Look for lines like:
script_filename = /home/example/public_html/wp-content/plugins/plugin-name/
This tells you exactly which plugin was running when PHP-FPM exceeded its performance threshold.
If you see repeated slow logs from the same plugin → remove or replace it.
Step 4: Monitor Resource Usage with htop
Open:
htop
Sort by CPU% or MEM%.
If PHP-FPM spikes to 100% when performing:
WooCommerce actions
Bulk edits
Import scripts
Custom plugin actions
Then the plugin responsible is running slow code.
Tip:
Watch in real-time while performing admin actions like:
Saving posts
Updating products
Running imports
Accessing plugin dashboards
You will quickly spot CPU-heavy plugins.
Step 5: Check MySQL Slow Query Log
Enable slow query log:
nano /etc/mysql/mariadb.conf.d/50-server.cnf
Enable:
slow_query_log = 1
long_query_time = 1
Restart:
systemctl restart mysql
Then view:
tail -n 50 /var/log/mysql/mysql-slow.log
If you see paths like:
.../wp-content/plugins/plugin-name/
Or unoptimized meta queries with high execution times, the plugin is likely the cause.
Database-heavy plugins include:
WooCommerce
wpDiscuz
SEO plugins
Membership plugins
Reporting/analytics plugins
Step 6: Disable Background Tasks (Cron Jobs)
Many slowdowns happen in the background, not during a page load.
Plugins that often trigger scheduled tasks:
Backups
SEO crawlers
WooCommerce analytics
Security scans
Email marketing plugins
Check scheduled events:
wp cron event list
Look for tasks containing plugin names that run too often.
Slow CRON tasks = slow server.
Step 7: Check for PHP-FPM Worker Exhaustion
Sometimes plugins overload memory, causing PHP-FPM to run out of workers.
Check worker usage:
systemctl status php8.1-fpm
Or:
ps aux | grep php-fpm
👉 If PHP-FPM continues to spike due to low memory, consider adding swap space — it can stabilize your server during plugin-heavy tasks.
Step 8: Compare Plugin Performance with Debug Bar
Another great tool is:
Debug Bar
Debug Bar Slow Actions
These show:
Slow hooks
Long-running functions
Plugins delaying the request lifecycle
This is especially useful for:
WooCommerce
Membership plugins
SEO plugins
Builders (Elementor, Divi, WPBakery)
When Should You Remove or Replace a Plugin?
Remove a plugin when:
It consistently shows in slow logs
It generates unoptimized DB queries
It triggers high CPU spikes
It delays admin dashboard loading
It causes PHP-FPM timeouts
It slows down cron jobs
You may need to replace it with lighter alternatives.
Conclusion
Identifying slow WordPress plugins requires a combination of WordPress tools and server-side diagnostics, but once you know where the bottleneck is, performance improvements become much easier.
Query Monitor, WP-CLI, PHP-FPM logs, and MySQL slow query logs provide a complete picture of what’s slowing down your site.
If you continue experiencing performance issues, the next step is analyzing MySQL bottlenecks, which we will cover tomorrow.
