Hi pmatos,
the blurring/refocusing effect you see is due to the progressive encoding technique used to increase the responsiveness. It can be disabled by checking the “Disable multi-pass display encoding” checkbox in the ‘Display settings’ tab from the menu panel. In the same panel you can tweak the Quality slider to find the better compromise between quality and performance.
The colors shift is due to the colorspace conversion required by the encoder. Find here the Feature Request addressing this issue: https://www.nomachine.com/FR03M02907.
Slowness depends certainly on the network bandwidth. Enabling the H.264 encoder would be a way to lower the used bandwidth. Here you can find the relevant info: https://www.nomachine.com/AR10K00706.
Finally, since you are connecting to a linux server, you may want to try the “lightweight” mode. That would help with bandwidth usage and would provide perfect quality for most graphics elements on screen. Here are the info on the lightweight mode: https://www.nomachine.com/AR02L00779.
In order to enable the lightweight mode, you need one of the Terminal Server products. Evaluation packages can be downloaded from this page: https://www.nomachine.com/terminal-server.