Mac Studio (M5 Ultra) with 256 GB of RAM review: a dream machine to run local AI agents and a massive leap over M3 Ultra for prompt processing and generation

For the past few days, I've been testing the (currently) top-of-the-line M5 Ultra Mac Studio with 256 GB of RAM.

MacStories 2026-09-22 Federico Viticci

Context & Ripple Effects

Apple positioned the M5 Ultra Mac Studio as its highest-end desktop option in August, pairing the chip family with up to 512GB of unified memory in its Mac Studio launch. That extends the workflow argument made for the M3 Ultra model's large-memory local-model workloads in 2025.

This review narrows the case to a 256GB configuration and prompt processing and generation, while another review flags that the M5 Ultra costs about twice as much as the M5 Max without nearly doubling performance. The split makes capacity-heavy local AI, rather than general benchmark leadership, the key reason to choose the top configuration.

First-order effects

Developers and power users running local agents gain a desktop configuration the review finds materially faster than M3 Ultra for prompt processing and generation, with 256GB of RAM available to hold larger workloads locally.
Apple's M5 Ultra Mac Studio becomes a more specialized purchase: its premium is most defensible for buyers whose local AI workloads need its memory and performance, rather than for users choosing between M5 Max and Ultra on broad performance alone.

Second-order effects

Apple's M5 Max tier faces a sharper value comparison, since the competing review's price-performance critique gives buyers a reason to reserve M5 Ultra spending for memory-bound local AI work.
Local-model software and agent workflows gain a higher-capacity Mac target, increasing the importance of memory-efficient model packaging and inference tools for Mac-based deployments.

Third-order effects

If local agents become a regular desktop workload, unified-memory capacity becomes a primary product differentiator alongside CPU and graphics benchmarks, reinforcing Apple's high-memory Mac Studio strategy.
The product points to a more heterogeneous AI-compute market in which local desktops complement hosted inference for workloads where users prioritize running models on their own machines.

The trend: AI demand is pushing edge-compute hardware toward memory-rich desktops designed to run increasingly capable local models and agents.

Discussion

@ekurutepe Engin Kurutepe on x
@viticci This is where the puck is headed. In a few years I think we'll all have an ultra-like machine powering our local models which we access from our mobile devices. I wonder if we'll really use MacBooks anymore.
@viticci Federico Viticci on x
@TheAhmadOsman 100% true and crazy that I can run Flash-Next on my desk with this performance right now. That model is so good. https://www.macstories.net/...
@ivanfioravanti Ivan Fioravanti on x
@MiaAI_lab @plotarmordev Same thinking! Wondering why all testers went for Qwen 3.8 Flash Next, but tomorrow we'll see other models in action!
@viticci Federico Viticci on x
oh, and Apple also sent me an M5 Pro Mac mini, 64GB of RAM I spent more time with the M5 Ultra, but for the price and size, this thing also rips! will do more testing on this soon
@viticci Federico Viticci on x
@MKBHD this is 100% spot-on. And if there's an area where 256 GB RAM (or even 512 GB) still ISN'T enough, that's local AI. give us more 😜
@thekitze @thekitze on x
@viticci This convinced me to get a 5090 instead of m5 studio lol
@viticci Federico Viticci on x
finally, shout out to GPT-6 Astra, which built a testing harness for local AI, then coordinated 2 Macs + 1 PC with remote connections and Computer Use over 3 DAYS Astra orchestrated ~140 local model runs across 3 machines. Wild thing to watch. https://www.macstories.net/...
r/LocalLLaMA r on reddit
M5 Ultra Mac Studio Review: The Dream Mac for Local AI Agents
@ivanfioravanti Ivan Fioravanti on x
Great review of the M5 Ultra by Federico and let's keep in mind these numbers are just the starting point, everything will just get better in the upcoming weeks! Can't wait to try one and buy the 512GB version!
@mweinbach Max Weinbach on x
I have been using the Mac Studio with M5 Ultra for the past week, and it's the most powerful computer I've used. Been a bit busy over the past week so my report will be slightly delayed, but just know... Qwen 3.8 Flash Next is the best model for the 256GB version, it rips
@viticci Federico Viticci on x
I ran all the M5 Ultra tests I could think of: - Two models (Flash-Next and GLM-5.3-Flash) - 4k, 8k, 16k, 64k, 128k, and 256k prompts - M5 Ultra vs M3 Ultra vs RTX 5090 - 4/5/6/8-bit Flash-Next quants I created all the possible charts and visualizations I could think of. Hope the…
@zoneoftech Daniel on x
The M5 Ultra Mac Studio is INSANE! Apple sent me the maxed out version, with a 36-core CPU, 80-core GPU and 256GB of RAM. I used it to run Deepseek v4 Flash (156GB model) fully on device to build a mini-game, which it did in just a few minutes! #MacStudio #M5Ultra @apple
@miaai_lab Mia on x
M5 Ultra's prefill speeds are up around 150% vs M3 Ultra when running Qwen3.8 Next Flash 🤯 Strong numbers! I wonder what the prefill speeds would look like with a model that has more active parameters. Qwen3.8 Flash Next has only 6B active.
@pcmag @pcmag on x
We got an early unboxing of the 2026 Mac Studio, and Apple packed serious internal upgrades into this compact workstation. 🤖🚨 Powered by either the M5 Max or the new M5 Ultra chip with up to a 36-core CPU and 80-core GPU, it delivers up to 4.3x faster local AI performance and sig…
@jundotkim Jun Kim on x
Great review from @viticci on the M5 Ultra Mac Studio. Glad oMLX could be part of it. Stay tuned for the next oMLX release!
@mkbhd Marques Brownlee on x
Been using the M5 Ultra Mac Studio for the past week and predictably it is the most powerful computer I've ever used. At this point, the improvements are not even targeted at me (a video creator) anymore. They're targeted at being the best machines for local AI work. There's noth…
@mweinbach Max Weinbach on x
The other thing I noticed was this was the first time where I could run a ton of subagents, with cloud models, and each had access to enough CPU resources that it wasn't really limited. It also compiled rust like 2x faster than M3 Ultra
@viticci.macstories.net Federico Viticci on bluesky
I ran all the M5 Ultra tests I could think of: — Two models (Flash-Next and GLM-5.3-Flash) — 4k, 8k, 16k, 64k, 128k, and 256k prompts — M5 Ultra vs M3 Ultra vs 5090 — 4/5/6/8-bit quants — I created all the possible charts and visualizations I could think of. …
r/apple r on reddit
Mac Studio (M5 Max) 2026 review: Shifting into overdrive - MacWorld
r/LocalLLaMA r on reddit
M5 Ultra Mac Studio Review: The Dream Mac for Local AI Agents - MacStories
@stevemoser @stevemoser on x
“One small wrinkle in the M6 variants the Mac mini uses: The basic 16GB Mac mini offers 153 GB/s of memory bandwidth, the same amount as the M5. The 24GB and 32GB versions offer 170 GB/s, an 11 percent increase that should marginally improve graphics performance and the speed of …
@chancehmiller Chance Miller on x
My review of the M6 Mac mini is now live. It keeps the same adorable design we've come to love, and the M6 chip is impressive. A few things have changed since the last Mac mini update, though. My full thoughts: https://9to5mac.com/...
r/apple r on reddit
Apple Mac mini (M6 2026) Review: Strong performance, but losing its grip on value - TomsHardware

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