I have found that my preferred purple track becomes more or less visible against brown roads depending on whether I view the screen through the top or bottom of my varifocals. Looking through the bottom (close focus) and the purple fades out. Looking through the top (distance but my handlebars now out of focus) and the purple is quite distinct. It is a progressive shift, not a step change. I think I might be able solve that particular problem with a different colour scheme.
I created a typ file to fade the map colours so the track stands out better. It's fiddly but seems to stick for subsequent updates.
https://www.openfietsmap.nl/tips-tricks/customizeI wonder if moving the maps to an SD card might free up memory and improve speed.