'Creating surface took WAY TOO LONG [. .]'

[ALinuxUser]

If I run Plank from the terminal, I read:

[WARN 08:23:49.981375] [SurfaceCache:196] Creating surface took WAY TOO LONG (35ms), enabled downscaling for this cache!

Explanation?

(I note also that, if one compiles Plank from source, one needs to ensure that files in /usr/local/share/plank/themes/Default are writable by current user, but have their execute bits unset. Otherwise one gets errors in the terminal.)

My system
=========

OS: Linux Mint 18.1 xenial x86_64
MMNso/` dMM `.::-. .-::.` .hMN: Kernel: 4.8.0-39-generic
ddddMMh dMM :hNMNMNhNMNMNh: `NMm Uptime: 6 hours, 5 minutes
    NMm dMM .NMN/-+MMM+-/NMN` dMM Packages: 3454
    NMm dMM -MMm `MMM dMM. dMM Shell: /bin/bash
    NMm dMM -MMm `MMM dMM. dMM Resolution: 1280x800
    NMm dMM .mmd `mmm yMM. dMM DE: Cinnamon
    NMm dMM` ..` ... ydm. dMM WM: Mutter (Muffin)
    hMM- +MMd/-------...-:sdds dMM WM Theme: Cinnamon (SimpleX-Grayscale_NJ)
    -NMm- :hNMNNNmdddddddddy/` dMM Theme: Mint-Y [GTK2/3]
     -dMNs-``-::::-------.`` dMM Icons: Paper [GTK2/3]
      `/dMNmy+/:-------------:/yMMM Font: Noto Sans UI 11 [GTK2/3]
         ./ydNMMMMMMMMMMMMMMMMMMMMM CPU: Intel 2 Duo T7500 (2) @ 2.2GHz
            .MMMMMMMMMMMMMMMMMMM GPU: Intel Integrated Graphics
                                     Memory: 3358MB / 7973MB

Plank 0.11.3, compiled from source.