GPT-4o vs Gemini 3.5 Flash: Latency and Throughput Compared

If you're picking a fast, general-purpose API for a chat product or an agent loop, GPT-4o and Google's Gemini Flash tier are two of the most common defaults. They sit in the same part of the market — quick, multimodal, cheap enough to run at volume — but they make different speed tradeoffs. This post lines them up on the metrics that actually decide user experience: time-to-first-token (TTFT), total latency, and throughput (tokens/sec).

One wrinkle to get out of the way first: Google's current fast model is Gemini 3.5 Flash, shipped at Google I/O on May 19, 2026. It's the successor to Gemini 2.5 Flash, which is what most older "GPT-4o vs Flash" comparisons (including our own April snapshot) actually measured. We'll be explicit about which generation each number comes from.

A note on data this week: ModelStats' live monitoring feed was unavailable while this post was written, so the figures below come from our last published snapshot (April 3, 2026) and from cited third-party sources, each labeled with its date. For current, real-time TTFT and tokens/sec across the models we track, check the live dashboard at modelstats.ai.

The two models, briefly

GPT-4o is OpenAI's flagship multimodal model — fast, widely deployed, and a known quantity in production. It's not the newest model OpenAI ships (the GPT-5 generation sits above it), but it remains one of the most popular API targets because it's well-understood and cheap relative to frontier reasoning models.

Gemini 3.5 Flash is Google's latest fast-tier model and the first in the 3.5 series. Google positioned it for throughput-heavy, agentic workloads and claims performance comparable to larger flagships at roughly 4× the speed of previous Gemini versions (Google). It's brand new, so treat any speed number attached to it — including the vendor's — as early.

TTFT: who starts responding first?

TTFT is the metric that makes an interface feel instant or broken. Sub-500ms feels immediate; past a second or two, users notice the wait.

From our last published ModelStats snapshot (April 3, 2026), captured from real API monitoring:

Model (April 3 snapshot)	TTFT	Total latency	Throughput
GPT-4o	758 ms	968 ms	not recorded
Gemini 2.5 Flash (predecessor)	492 ms	696 ms	74.73 tok/s

In that snapshot, Google's Flash tier started responding meaningfully sooner than GPT-4o — 492ms vs 758ms — and finished a full round-trip faster too. That was Gemini 2.5 Flash, the prior generation. We don't yet have a committed ModelStats TTFT figure for Gemini 3.5 Flash, so we won't put a number on it we haven't measured. Given that 3.5 Flash is pitched as faster than its predecessor, the gap over GPT-4o is plausibly at least as wide — but that's an expectation to verify, not a measurement.

Throughput: who finishes a long answer first?

TTFT only tells you when generation starts. For long outputs — summaries, code, agent reasoning — what matters is tokens/sec, which determines total wall-clock time.

This is where the Flash tier has historically pulled ahead. In the April 3 snapshot, Gemini 2.5 Flash led the entire tracked field at 74.73 tokens/sec. GPT-4o's throughput wasn't recorded in that snapshot, so we can't give it a head-to-head number from our own data — but it has generally trended well below the Flash tier's peak in our monitoring.

For Gemini 3.5 Flash specifically, the public numbers diverge sharply depending on who's measuring: Google's own API figures land around 175 tokens/sec, while independent benchmarker Artificial Analysis measured north of 280 tokens/sec (Artificial Analysis). That ~1.6× spread between vendor and independent figures is itself the lesson: throughput depends heavily on prompt shape, output length, region, and measurement methodology. Whatever the exact number, 3.5 Flash is firmly in the high-throughput camp, which is the whole point of the Flash line.

How to read the tradeoff

The honest summary, grounded in what we can actually cite:

• For perceived speed in chat, the Gemini Flash tier has the TTFT edge — it started ~270ms sooner than GPT-4o in our April snapshot, and 3.5 Flash is built to be faster still.
• For long-output and batch work, throughput dominates total time, and the Flash tier is one of the highest-throughput options on the market. GPT-4o is a solid, predictable workhorse but not the throughput leader.
• For ecosystem fit, the choice often comes down to which platform you're already on — tooling, multimodal needs, and existing prompt investments usually outweigh a few hundred milliseconds.

A few hundred milliseconds of TTFT difference is real but rarely decisive on its own. Measure it against your prompts, in your region, before committing — vendor and independent numbers already disagree by more than 1.5× for the newest model here, which tells you how much your own traffic conditions matter.

See it on live data

Numbers shift throughout the day with load and routing. To compare these two on current monitoring data rather than a static snapshot, see the head-to-head page: GPT-4o vs Gemini 3.5 Flash. It plots TTFT, latency, and throughput from continuous pings every 10 minutes, so you can watch how the gap actually behaves over a week rather than trusting a single capture.

Takeaways

• TTFT: In our April 3, 2026 snapshot, the Gemini Flash tier (2.5 Flash, 492ms) started faster than GPT-4o (758ms); 3.5 Flash is pitched to be quicker still — verify, don't assume.
• Throughput: The Flash tier is a throughput leader (74.73 tok/s for 2.5 Flash in our snapshot); third-party figures put Gemini 3.5 Flash between ~175 and ~280 tok/s depending on methodology.
• GPT-4o remains a fast, predictable, widely-supported default — strong on consistency, not the raw speed leader.
• Measure on your own traffic. Track real, current numbers for both at modelstats.ai, updated every 10 minutes.