MacBook Pro 15" 2015 iGPU slows down from normal 750mhz to 200mhz I've faced with issue when sometimes after leaving sleep mode the iGPU is going into 200mhz instead normal 750mhz. The UI works slow, especially while you're switching desktops, scrolling in WebStorm and so on.

Here is some thermal data:

$ sudo thermal levels
cpu: 0
gpu: 0
io: 0


And some powermetrics:

$ sudo powermetrics -s gpu_power
Machine model: MacBookPro11,4
SMC version: 2.29f24
EFI version: 187.0.0
OS version: 18D109
Boot arguments:
Boot time: Mon Feb 11 13:59:37 2019



*** Sampled system activity (Tue Feb 12 17:39:04 2019 +0200) (5000.23ms elapsed) ***


**** GPU usage ****

GPU 0 name IntelIG
GPU 0 C-state residency: 98.22% (0.43%, 97.79%)
GPU 0 P-state residency: 1200MHz: 0.00%, 1150MHz: 0.00%, 1100MHz: 0.00%, 1050MHz: 0.00%, 1000MHz: 0.00%, 950MHz: 0.00%, 900MHz: 0.00%, 850MHz: 0.00%, 800MHz: 0.00%, 750MHz: 0.00%, 700MHz: 0.00%, 650MHz: 0.00%, 600MHz: 0.00%, 550MHz: 0.00%, 500MHz: 0.00%, 450MHz: 0.00%, 400MHz: 0.00%, 350MHz: 0.00%, 300MHz: 0.00%, 250MHz: 0.00%, 200MHz: 1.78%
GPU 0 average active frequency as fraction of nominal (200.00Mhz): 100.00% (200.00Mhz)
GPU 0 GPU Busy 1.78%
GPU 0 FB Test Case 0
GPU 0 [PSR] GPU + TCON are Off    : 0.00%
GPU 0 [PSR] Only GPU is On        : 100.00%
GPU 0 [PSR] Only TCON is On       : 0.00%
GPU 0 [PSR] GPU + TCON are On     : 0.00%
GPU 0 [PSR] Enter Count           : 0 (0.00/second)
GPU 0 [PSR] Abort Count           : 0 (0.00/second)
GPU 0 DPB on 0.00%
GPU 0 GT2 on 0.00%
GPU 0 GT3 on 100.00%


And here we go: 200MHz: 1.78%

And let me show iStat Menus output:



And only reboot is helpful. After reboot iGPU returns to 750mhz and UI works great.
Is it the Apple's bug? Someone faced with the same issue? 
It's only about MacBook Pro 15" 2015 !! I have this issue on both (home laptop and on my work).
Looks like I've faced it only on Mojave..
The new 10.14.6 macOS update says it "resolves a graphics issue that may occur when waking from sleep". I'm hoping that might be referring to this issue (I filed an issue a few weeks ago at https://feedbackassistant.apple.com/, so that may have prompted them to look into it, though they haven't responded to it).

I'll update this answer if I see the problem again after the update.

UPDATE: I'm still seing the problem in 10.14.6 (as well as OP who commented below), so this is unfortunately not the solution.