Hi Miro,
If there are so many items loaded into a drop down list control, the performance wont be good, as well as the user experience.
As a workaround, I would suggest you create something like a cascade drop down to filter the data you want using several drop down list controls, it will help to avoid
loading too many items into one page, the page will not have to stuck for such a long time .
By doing this, a possible scenario will be like this: suppose that there is data model like Country-Region-State, we need to only select a State from a drop
down list. With the help of a cascade drop down, we can select the corresponding Country. Then there will be a subset of Region retrieved from the server. Select the corresponding Region, then a subset of State retrieved, we can easily select the specific
State within a small range of items finally.
About how to implement a cascade drop down in SharePoint, the demo below would be helpful:
https://spservices.codeplex.com/wikipage?title=%24().SPServices.SPCascadeDropdowns&referringTitle=Documentation
Thanks
Patrick Liang