The only possibility would be to use a Threadpool and call it every X times. That packet DRAWS things on client, there is nothing like "realtime". It stays on screen until you decide it mustn't stay on screen.
As you say, more you use forms, laggier your client will be. At this moment, I don't think it's possible to dump all the "hidden" forms, even after a packet reset. Client properly cleanups when the Player actually disconnects to the lobby, which probably cleans all forms from client memory.
Nobless cardinal with subclass:
Nobless Mystic Muse with subclass:
x2 dc set robe set
x1 tlm robe set
x1 mj robe set
1 arcana mace
lots of mats and 500kk
ALL TOGETHER 60EURO