Very slow screen refresh

[dca00]

When I drag the map, it takes a very long time to draw.
Is this poor graphics performance or reading from the map file?
If it is the latter, could you read and cache areas around the currently displayed area, forming a square like this:

xxx
xox
xxx

where o is current screen and xx are same size areas adjacent to o? Then you will not have to wait while you read them from media.
Because it is painfully slow. Too slow to be user-friendly in 2024. We could have probably tolerated such slowness on 8088 back in 1980s.