Dell P2415Q 4K monitor gets 60Hz by itself. but when a second display is connected, it drops to 30Hz

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Dell P2415Q 4K monitor gets 60Hz by itself. but when a second display is connected, it drops to 30Hz

I'm experiencing some really weird problem here.

So I have been using this Dell P2415Q 4K monitors with my MacBook Pro 16'' for a while now, all good, 4K 60Hz no problem.

But recently I got a second display: Dell U2719DC. When I plug that display in, my Dell P2415Q 4K monitor drops to 30Hz.

I don't get it. Supposedly, the MacBook Pro should be able to power two 4K monitors no problem. Why would plugging in an extra display drops the frame rate?

MacBook Pro with Touch Bar

Posted on Jul 14, 2020 11:13 PM

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Answer 1

BDAqua

Level 10

250,259 points

Jul 15, 2020 9:59 AM in response to Jim Zenn

Which exact MBP? What exact connectors?

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Answer 2

Jim Zenn Author

Level 1

9 points

Jul 15, 2020 11:52 AM in response to BDAqua

Thanks BDAqua for your quick response, I really appreciate it.

MacBook Pro ver:

MacBook Pro (16-inch, 2019)

2.6 GHz 6-Core Intel Core i7

AMD Radeon Pro 5300M 4 GB

Intel UHD Graphics 630 1536 MB

Displays and connectors:

DELL P2415Q (via displayport -> displayport to usb-c adapter [StarTech CDP2DP] -> MacBook Pro)

DELL U2719DC (via displayport -> DELL WD19TB Dock -> MacBook Pro)

Again. To emphasis, when connecting DELL P2415Q with this connection on its own, the display can reach 4K 60Hz no problem. As can be seen from System Report > Graphics/Displays , logged below as additional text.

System Report: when only DELL P2415Q is connected

But when I plug in the second display, DELL U2719DC, the frame rate of DELL P2415Q drops to 30Hz. As can be seen from System Report > Graphics/Displays , logged below as additional text.

System Report: when both displays are connected

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Answer 3

BDAqua

Level 10

250,259 points

Jul 15, 2020 4:46 PM in response to Jim Zenn

Yeah, supposedly...

2nd Display Support:Dual/Mirroring*2nd Max. Resolution: 6016x3384 x2*

Details: *In addition to simultaneous support of the internal display, this model supports a maximum resolution up to 6016x3384 at 60 Hz at over a billion colors on up to two external displays or up to 4096x2304 resolution at 60 Hz at over a billion colors on up to four external displays via Thunderbolt 3 (USB-C).

I wonder if it's really using the Dedicated GPU?

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Answer 4

Jim Zenn Author

Level 1

9 points

Jul 15, 2020 7:03 PM in response to BDAqua

Sure looks like they are both using the dedicated GPU. They both shows below the AMD Radeon Pro 5300M tab.

Another thing that's worth mentioning: actually there were one or two times both monitors were working at 60Hz! However, after a reboot or two, it's gone :(

Feels like a software bug to me. I wonder if I can report it.

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Dell P2415Q 4K monitor gets 60Hz by itself. but when a second display is connected, it drops to 30Hz

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