How much faster is the RTX 4090 than the 3090 for LLMs?

936 vs 1008 GB/s — theoretically ~8% faster decode: 8B Q4 ~143 vs ~154 tok/s, Qwen3 32B Q4 ~35 vs ~38 tok/s. Barely perceptible in chat; the real gap is price, not speed.

Is a used 4090 worth over twice a used 3090?

Not for pure LLMs: same 24GB, identical verdict matrix — the $2,400 vs $1,100 gap buys zero new model coverage. The 4090's premium comes from discontinuation, gaming, and the 5090 anchor, not LLM value.

Which card is better for a dual-GPU 70B build?

The 3090. Two used cards (~$2,200) give 48GB with ~1591 GB/s aggregate bandwidth; dual 4090s cost ~$4,800 for the same 48GB. Paying double for +8% makes no sense. Mind PCIe lanes and an 850W+ PSU.

GPU Comparisons / RTX 3090 vs RTX 4090

RTX 3090 vs RTX 4090 for Local LLMs: Which Should You Pick

Verdict

For pure LLM inference, buy the 3090: same 24GB, an identical verdict matrix, bandwidth only 8% behind, at less than half the 4090's used price. The 4090 buys convenience, efficiency, and gaming.

Spec Comparison

	RTX 3090	RTX 4090
Nominal VRAM	24 GB	24 GB
Usable VRAM (for models)	24.0 GB	24.0 GB
Memory bandwidth	936 GB/s	1008 GB/s
Type	Discrete GPU	Discrete GPU
Release year	2020	2022
MSRP	$1,499	$1,599
Used reference price	$1,100	$2,400
TDP	350 W	450 W

Model Verdict Matrix (Q4_K_M @8K)

Model	VRAM needed	RTX 3090	Theoretical speed	RTX 4090	Theoretical speed
Llama 3.2 3B	4.4 GB	Comfortable	≈357	Comfortable	≈385
Llama 3.1 8B	7.5 GB	Comfortable	≈143	Comfortable	≈154
Qwen3 8B	7.7 GB	Comfortable	≈140	Comfortable	≈151
Phi-4 14B	12.2 GB	Comfortable	≈78	Comfortable	≈84
Mistral Small 3.2 24B	17.5 GB	Comfortable	≈48	Comfortable	≈51
Gemma 3 27B	22.4 GB	Tight fit	≈42	Tight fit	≈45
Qwen3.8 27B	20.7 GB	Tight fit	≈41	Tight fit	≈44
Muse Glimmer 30B	20.1 GB	Tight fit	≈39	Tight fit	≈42
Qwen3 30B-A3B	21.0 GB	Tight fit	≈347	Tight fit	≈374
Qwen3 32B	23.7 GB	Tight fit	≈35	Tight fit	≈38
gpt-oss-20b	14.7 GB	Comfortable	≈318	Comfortable	≈343
Llama 3.3 70B	47.4 GB	Not feasible	—	Not feasible	—
gpt-oss-120b	73.6 GB	Not feasible	—	Not feasible	—
DeepSeek-R1 671B	413.1 GB	Not feasible	—	Not feasible	—

Highlighted rows = watershed models where the two cards' verdicts differ. Theoretical speed = bandwidth × 0.75 ÷ per-token weight bytes (active params for MoE); real-world results vary with framework/driver/CPU, ±30%. Speed is hidden for multi-GPU/infeasible verdicts.

Value Comparison

	RTX 3090	RTX 4090
Bandwidth per dollar	0.85 GB/s/$	0.42 GB/s/$
Usable VRAM per dollar	0.022 GB/$	0.010 GB/$

Pricing basis: RTX 3090 = used reference price; RTX 4090 = used reference price; used prices are market estimates, see analysis for volatility.

Analysis

This is the most-asked comparison in local LLM circles, and the answer is simple: both cards have 24GB, so the 14-model verdict matrix is identical row by row — the 24B tier is comfortable (Mistral Small Q4 ~17.5GB), the 27-32B tier is tight (Qwen3 32B ~23.7GB), and 70B is infeasible on either single card. The only difference is speed: 936 vs 1008 GB/s means decode is theoretically ~8% apart — 8B Q4 at ~143 vs ~154 tok/s, Qwen3 32B Q4 at ~35 vs ~38 tok/s. In daily chat you can barely tell.

The price gap is the real watershed: in August 2026 a used 3090 runs $1,095-1,181, while the discontinued 4090 commands $2,350-2,500 — over 50% above its $1,599 MSRP, its price anchored to the RTX 5090. That's a 2.2× spread. Bandwidth per dollar is 0.85 vs 0.42 GB/s/$ — the 4090 delivers half the 3090's LLM value. One used 4090 costs what two 3090s cost, and the pair's 48GB unlocks 70B Q4 outright. That is the rational answer from an LLM perspective.

The 4090's legitimate profile: you want exactly one card, care about noise and per-token energy, and game or create on the side; or you'd rather not deal with a 2020 card's mining history and GDDR6X thermal pads (its own 450W TDP and 12VHPWR connector have their own inspection list). Verdict: budget-driven pure-LLM buyers take the 3090 and save ~$1,300 toward a second card; experience-driven one-card buyers won't regret the 4090 — they're just paying for things LLMs don't use.

FAQ

How much faster is the RTX 4090 than the 3090 for LLMs?: 936 vs 1008 GB/s — theoretically ~8% faster decode: 8B Q4 ~143 vs ~154 tok/s, Qwen3 32B Q4 ~35 vs ~38 tok/s. Barely perceptible in chat; the real gap is price, not speed.
Is a used 4090 worth over twice a used 3090?: Not for pure LLMs: same 24GB, identical verdict matrix — the $2,400 vs $1,100 gap buys zero new model coverage. The 4090's premium comes from discontinuation, gaming, and the 5090 anchor, not LLM value.
Which card is better for a dual-GPU 70B build?: The 3090. Two used cards (~$2,200) give 48GB with ~1591 GB/s aggregate bandwidth; dual 4090s cost ~$4,800 for the same 48GB. Paying double for +8% makes no sense. Mind PCIe lanes and an 850W+ PSU.

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Data verified 2026-09-01