Beirit Harvey

Most LLM performance advice focuses on model choice and GPU counts. This blog jumps into the top five bottlenecks teams run into once real users show up, plus straightforward ways to fix them.

LLMs in Production: The Top 5 Bottlenecks Nobody Mentions

Most LLM performance advice focuses on model choice and GPU counts.

In production, the problems are usually simpler (and more frustrating): memory fills up, requests pile up, inputs take too long, systems trip over each other, and small updates quietly break performance.

This blog jumps into the top five bottlenecks teams run into once real users show up, plus straightforward ways to fix them.


1) Memory Gets “Full” Faster Than You Expect

What you see

 * It works fine in testing, then slows

Most LLM performance advice focuses on model choice and GPU counts.

In production, the problems are usually simpler (and more frustrating): memory fills up, requests pile up, inputs take too long, systems trip over each other, and small updates quietly break performance.

This blog jumps into the top five bottlenecks teams run into once real users show up, plus straightforward ways to fix them.


1) Memory Gets “Full” Faster Than You Expect

What you see

 * It works fine in testing, then slows down or crashes under real traffic.
 * Long prompts or high concurrency cause sudden instability.

Why it happens
LLMs need extra memory to keep track of what they have already read. As prompts get longer and more people use the system at once, that memory grows quickly.

How to fix it

 * Set clear limits on prompt length and concurrency (so one heavy request doesn’t sink the system).
 * Split traffic: keep long-context requests separate from normal chat requests.
 * Watch memory headroom, not just “GPU utilization.” If you’re constantly close to the edge, you will see chaos.


2) Throughput and Latency Fight Each Other

What you see

 * You can make it fast for one user, or efficient for many users, but not both.
 * p95 latency spikes when traffic increases, even if average performance looks OK.

Why it happens
To run efficiently, systems group requests together. But waiting to group requests adds delay. So you’re always balancing “serve immediately” vs “serve efficiently.”

How to fix it

 * Put a hard cap on how long requests are allowed to wait before running.
 * Use two lanes:
   * Interactive lane: lower delay, consistent p95
   * Batch lane: higher efficiency for non-urgent work
 * Measure “time to first response” separately from “overall speed.” They are different problems.
 * Benchmark and test with real-life traffic distributions rather than static input/output values, to better model real world performance and accurately plan for compute needed to meet throughput and latency requirements


3) The “Before the Model Runs” Steps Become the Bottleneck

What you see

 * The system feels slow, but the GPU charts don’t look maxed out.
 * Long prompts feel disproportionately slow.

Why it happens
A lot happens before the model generates the first token: preparing the prompt, processing the input, and getting the request ready to run. With long prompts, this can dominate total time.

How to fix it

 * Cache repeatable prompt parts (system instructions, templates, tool definitions).
 * Keep your input pipeline simple. Every extra step adds time.
 * Treat “very long prompts” as a separate workload with different rules.


4) Things Collide: Network, Storage, and “Noisy Neighbors”

What you see

 * Performance is inconsistent. Some hours are fine, others feel randomly slow.
 * Jobs slow down when multiple workloads run at once.

Why it happens
LLM systems are not just compute. They also depend on the network and storage. If those layers get crowded, performance becomes unpredictable.

How to fix it

 * Don’t mix everything together.
   Separate:
   * production serving traffic
   * big data movement
   * large training jobs
 * Track performance over time, not just averages. In production, variance is often the first warning sign.


5) Small Updates Quietly Cause Big Slowdowns

What you see

 * “Nothing changed” but the system is slower this week.
 * A new driver, framework update, or container change causes a drop in performance.

Why it happens
LLM performance is sensitive to software versions. Tiny upgrades can shift speed, stability, or memory behavior.

How to fix it

 * Pin versions (drivers, libraries, framework) for production.
 * Roll out changes gradually.
 * Keep a simple weekly benchmark with your real prompt sizes so regressions show up immediately.


What “Good” Looks Like

A production LLM setup is in a good place when you can answer these quickly:

 * Are we slow because of memory, queueing, inputs, network/storage, or a recent change?
 * Which users or endpoints are driving the worst slowdowns?
 * Did performance change after an update?

If you can’t answer those yet, that’s normal. It just means the next step is making performance visible.


Closing

LLMs don’t fail in production because teams didn’t buy enough GPUs. They fail because the system around the model gets overwhelmed in predictable ways.

Fix the bottleneck you actually have, and performance usually comes back fast.


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LLMs in Production: The Top 5 Bottlenecks Nobody Mentions

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