Diagnosing a slow Rails page, layer by layer

"This page feels slow" is a vague bug report. Before changing any code, it helps to
have a fixed way to locate where the time goes. A Rails request passes through a few
predictable layers, and each layer has its own tools and its own typical failure mode.

Here is the mental model I use, and a real example of walking it end to end.

The layers of a request

Middleware → Controller → SQL → View → external calls → browser

Most slowness lives in one of these:

The point is not to guess. It's to read the numbers and let them point at the layer.

Step 1 — read the numbers (development)

In development I lean on two tools:

• rack-mini-profiler — a badge in the corner that breaks a request into controller / view / SQL time, and counts every query.
• bullet — watches for N+1 and tells you exactly which association to preload.

On a storefront page that lists a page of 48 products, rack-mini-profiler showed:

Executing: stores#show     2 sql
Rendering: show.html.erb   49 sql   ← 49 queries just to render?
SQL Summary:               51 sql total

Fifty-one queries to render one page of products is a red flag, and the fact that 49 of
them happen during rendering points straight at the view. bullet confirmed it:

USE eager loading detected
Product => [:image_attachment]
Add to your query: .includes([:image_attachment])

Step 2 — understand the N+1

The products use Active Storage for their images:

class Product < ApplicationRecord
  has_one_attached :image
end

An attached image isn't a column on products. Active Storage spreads it across three
tables: active_storage_attachments (which record owns which file),
active_storage_blobs (the file's metadata + a storage key), and
active_storage_variant_records (generated thumbnails). The file bytes themselves live in a storage service — disk locally, object storage in production.

So every time the view touches product.image, Rails walks those tables. In a loop over N products, that's N extra round-trips: a textbook N+1.

Step 3 — fix and re-measure

The fix is to preload the attachment once, up front. Active Storage generates a scope
for exactly this:

# before
@products = Product.where("stock > 0")
# after
@products = Product.where("stock > 0").with_attached_image

(with_attached_image is just an Active Storage flavoured includes.)

Re-measured on the same page:

The query count is now flat regardless of how many products are on the page — O(1) instead of O(n). That's the real win: N+1 isn't scary because of its cost on any single request, it's scary because it grows with your catalog and your traffic. The same page under a few hundred requests a minute turns a handful of extra queries into thousands of extra round-trips against the database.

The other kind of slow: a single heavy query

N+1 is about query count. The other common case is one query that is slow on its own — usually a missing index. Here EXPLAIN is the tool: it shows how Postgres plans to run a query without running it.

Looking up orders by a column with no index, over an orders table with ~800k rows:

Seq Scan on orders  (cost=0.00..21450.00 rows=1 width=93)
  Filter: (customer_email = 'someone@example.com')

Seq Scan means Postgres reads the whole table row by row to find one order — wasteful when there are hundreds of thousands of them. After adding an index on that column:

Index Scan using index_orders_on_customer_email on orders  (cost=0.42..8.44 rows=1 ...)
  Index Cond: (customer_email = 'someone@example.com')

Seq Scan → Index Scan, and the planner's cost estimate drops from ~21,000 to ~8 — the index turns "scan everything" into "jump straight to the row."

Two gotchas worth knowing:

• Data volume matters. On a small table Postgres picks Seq Scan even when an index exists — scanning a few rows is cheaper than an index lookup. The index only earns its keep once the table is large, so test against production-scale data, not a dev seed.
• Selectivity matters. An index only helps when the query matches a small slice. A query that returns most of the table will be a Seq Scan regardless.

How this plays out in production

Development tools (rack-mini-profiler, bullet) catch problems before they ship. But dev never fully mirrors production — you don't hit every page, your data is small, and some N+1s only appear with real data shapes. So production needs an APM (I use Scout) watching real traffic to catch what slipped through.

The end-to-end flow when something is slow in production:

APM flags a slow endpoint
  → reproduce locally with realistic data
  → EXPLAIN the suspect query
  → add the index / preload / cache
  → deploy, confirm in the APM that it actually got faster

bullet is the prevention, the APM is the evidence. They're not redundant — they're defense in depth, because dev can never be a perfect copy of prod.

Takeaways

• Don't guess where a page is slow — read it layer by layer.
• Two distinct DB problems: many queries (N+1, fix with includes/preload) vs one slow query (missing index, find with EXPLAIN).
• N+1 matters because it scales with data, not because it's slow today.
• EXPLAIN results depend on data volume and selectivity — test with realistic data.
• Prevent in development, verify in production.