very slow loading time of `dashboard` requests

requests for data for a charts take too much time...
Repro Steps:
1. open the site (at dashboard page)
2. open chrome dev tool
3. look at at `network` panel

you will see it takes 10(!) seconds
it is lot of time!
for example - the `gtfs_agencies/list` request takes less than a second:

Start at the dashboard page and use Chrome DevTools' Network panel to identify which chart-data requests take about 10 seconds. Compare them with the faster `gtfs_agencies/list` request and trace the relevant request handling in the Python project. Done means the dashboard requests load substantially faster and the improvement is verified in the Network panel.

Written by the indexing model from the issue text.