Shimmering on a 64x64 display with ESP8266

[nygma2004]

I have a few project with this library, but this is the first time I am driving an 64x64 display. I am using an ESP8266.
When I display a full image (nothing moves, just a static image), sometimes I see some shimmering on the display, or occasional flicker.

Any tips on how to make the display stable? Could this be the limit of the 8266?
There is HTTP server implementation in the sketch as well, also MQTT handling, so those could be the reason as well.

I store the image in memory and use a single display.drawRGBBitmap(0, 0, forest, 64,64); command to display it.

I tested it with a bench power supply and the current stops at 1A. ESP is clocked to 160 MHz.