It should be possible to convert an encyclopedia book to the dictionary format (albeit probably quite some effort would be needed). I have done it (not for an encyclopedia but in principle that should not matter, just take longer) and described the way I did it here:
https://www.mobileread.com/forums/sho...d.php?t=119226
Regarding the speed, my biggest dictionary has ~40 MB size, 100.000 entries + a few hundreds of thousands inflections and the lookup is not really instant but it is acceptable (approx. 2-5 seconds). The number of entries+inflections should matter more than the size of the file for the speed of the lookup, so I imagine it should not be a major problem for an encyclopedia. Clearly, some entries would span over many pages in an encyclopedia, again not a major issue.