External HDMI Monitor Slows Down Macbook Pro to Unbearable Speeds?

I think the solution could be to use Mini DisplayPort to HDMI Cable. I'm going to validate this and revert. For now, I'm shutting down the MBP lid and using the external monitor. Works well.

My MBP 2015 with 16GB RAM didn't have this problem until upgrading to Sierra, which also coincided with using an external monitor at home. Thinking about it, I haven't had any issue with the Dell external monitor at the office, but it occurs at home with the new monitor.

The difference with both the setups are the cables. The HDMI to HDMI cable at home is problematic. The Mini DisplayPort to HDMI Cable at office works without any issues.

I have/had the same issue that my computer slowed down dramatically when I hooked it up to my external display.

So i read your comment about the different power source. And I must admit, it must be the stupidest thing I have ever read this year. I know its only 4th of January, but still.

So I go ahead and try it anyway, because thats the sort of time I have, had a very old power adapter which needs a converter etc... And it seems to work. Illustrator, photoshop etc... fast – well, normal speed.

Wow. who would have thought. Thanks.

If the system usage is spiking up that high, that's usually because of a sensor problem on the logic board. This could either be because the actual sensor failed, or that the adapter has failed or is drawing too much power and the computer is slowing itself down to prevent damage.

Try using a different adapter and if the problem persists, try taking your computer to an AASP that services older products. Due to the age of your device, Apple considered it vintage and the Apple Stores will no longer service it.

Just to add to this: I've got the same problem with connecting HDMI. I noticed tis with my 2012 Retina MBP after updating to Sierra. I never found a viable solution. Using Thunderbolt to HDMI or DVI worked fine, the HDMI out would bring the 2012 to its knees. I have a new MBP (2017) running High Sierra, and I tried using a USB-C to HDMI connection for a protector in class. The computer became so slow I couldn't open and change the display preferences. This is a persistant problem and it would be great to find a consistant solution.

More on this: I have been working for a week with the monitor switched to 720p and the slowdown effect stops completely. I find using my HD 1080p monitor (actually, a 32in samsung TV, about 5 years old but rather great as a monitor which I have used over HDMI for years) is incredibly useful. Lots of things being done at the same time as I am a developer. It seems this is a threshold issue. The Macos update or another update probably tipped the memory problem into focus. And the screen focus is affected by the frequency - the Mac is a macbook Pro with 16G ram. Not latest but good enough to drive everything. The problem is not the mac - it is the HDMI cable queuing up frames which is increasing kernel_task (which manages 165 threads, so creates a system bottleneck) CPU use to over 100% (multiple core measurements in Activity Monitor seem to show CPU load pretending there is one CPU core, I think, so over 100% utilisation is possible). After a week working with 720p without problem, I thought of changing it back to 1080p but at a lower frequency (50hz instead of 60hz) and the System overload has not yet occurred. I think the HDMI refresh can suddenly become demanding (probably a video triggers the condition) on the system and if the monitor can not sync with the system refreshes it seems to get into a CPU bound queue state.

Earlier in the thread, the Apple Support person took a "can not reproduce" stance which is understandable. It is a hardware design issue. The HDMI protocol suddenly works against you. A bit like trying to tow a boat with a car that does not have a big enough engine, it works fine downhill. The problem is a design issue with how HDMI works, the computer is being held up by the monitor/hdmi not being able to cope and the OS does not cope with the condition that can arise and as it overloads the kernel everything (especially Chrome) is affected.

This is an issue that could be addressed in the Kernel code but as it requires a set of conditions that you only see "in the field" - it is hard to reproduce. But hardware engineers may need to review the HDMI driver so that if it starts to stack up frames they get dropped rather than being allowed to queue if the kernel_task starts to use over 50% of CPU or some similar indicator... (this is not a spec). HDMI 1 is able to transport at 60hz 1080p and the next generation of HDMI can go up to 340hz. It is not a capability or cable quality issue. It is a protocol issue when the OS is being asked to tow something uphill and its coping mechanism kills the OS. It is an intermittent hardware issue.

My current test is to run it without video at 50hz and see if my productivity improves as I can use the full screen even if the slower refresh is slightly annoying...

I'm having the same issue with Thunderbolt to HDMI on a 2012 MBP. It's a Dell 27", nothing fancy, but the operation speed of Lightroom is WAY below what it is when I'm doing the work on the laptop alone. Really annoying still. And it sounds like switching to a straight HDMI isn't going to help either....but I'll still try this and see.

Well, it is actually for my sister's brand new 2017 13 inch macbook pro that I setup for her. Also, could it have anything to do with the power adapter being plugged into a wall outlet that I have a lot of things running thru....?

His IS a common complaint. I see it all over the internet on several different blogs and websites.

Finally Fixed:

mac ran fine most of time but CPU spiked to almost 100% cpu use and slowed to unusable with video playing in external monitor.

It was a heat issue. opened up the laptop (needed special screwdriver P5) cleaned out vents and fans for the first time in 3 years. dust was caked in. Easy to clean with a can of air, and clean toothbrush for fans.

Runs like a champ now. 3 screens no problem 10% CPU.

After OS upgrade to MacOS Sierra 10.12.6, I suddenly experienced extreme system slowdown (with no real reason) and then a failure of Vagrant. Installed a bunch of fixes but then noticed vagrant was only crashing when the HD resolution was being used by an HDMI cable. Weird. But removing the HDMI screen and the laptop was again fast. Can recreate. Possibly hardware fault exacerbated by software update. Or a fault in the HDMI 1080p/i drivers. Or something like that. Probably as HDMI cable is okay with 800 x 600 res. Better than nothing. Was definitely working before the upgrade but other upgrades also occurred in the same timeframe.

Yours is not a common complaint. The computer should not slow at all due to the external display.

I expect some other issue is at play here.

What Applications are you running? How much RAM memory is in the machine? How much is free?

Use Activity Monitor to check memory usage:

Activity Monitor: View system memory usage

When you are confident in that information, especially that Pageouts are not killing your performance, change to this display:

Runaway applications can shorten battery runtime

Wow, that's amazing.

Any time a problem is obvious and repeatable, the store can be helpful.

They put their hands on these machines all day every day, so they may have seen this before.

Are you running an unusual screen saver?