javascript - non blocking frameset (async) -

i'm not here argue fact still use frameset part of our application replaced (famous last words) time being want increase performance.

if take simple example of have:

<frameset cols="25%,*">   <frame src="frame_a.htm">  <frame src="frame_b.htm"> </frameset>   

the point frame_b gets loaded after frame_a ready. tho 'network' tab in every browser prove different, frames load 1 one. if load frame_a take 0.5 seconds, if load frame_b take 0.7 seconds. yet if load both above example, frame_a still take 0.5 seconds, while frame_b takes 1.2 seconds (0.5+0.7) load. hence assume browser 'blocks' request until frame_a ready.

now have tried multiple things. tried onload stuff js/jquery yet no matter try end same result.

is there way load "async"? , not mean load 'main' page first (that includes frameset) frames (a , b) gets loaded async total execution time gets halved?

i cannot change lot of code, if can figure out way 'easy' replace these frames while keeping exact functionality, ill happy :)