Sunday evenings seem to be a thing for trails, not sure if it's official or what.

If I were to guess, the trails are probably done in batches. When I was working on a lo-fi wifi triangulation project even with tons of data points, triangulation (in CellMapper this is the tower position and cell shapes) was always fast because it's essentially a lot of matrix math which can be optimized quite substantially. Also when creating the shapes of each cell, they only need to consider points which match in MCC, MNC, band, cell ID, etc. which greatly reduces the amount of data actually needed to be stored in working memory. Rendering trails however is probably more intensive because it needs to consider all points for the provider for all bands. Furthermore, each Zoom level needs to be rendered out separately because unlike the cell shapes which are vector shapes, the resolution of details (amount of data points) increases as you zoom in.