I've been thinking about script similar with
Enhanced merit UI [1.1], but showing total merit sent & count of merit transaction to the merited user from user's merit summary (which means total & count in last 4 months)
My concern is loading time to obtain and process the data, is it possible to make such script but works quickly?
With an API that returns the merit data and if the script is only activated when you click on an specific user rather than preloading it for every user on the page, then yes.
The main problem is probably getting the merit data. You would need to make your own scraper to get merit data and then build an API or maybe ask Vod or Loyce for an API.