You could maybe find out where the delay is by using st's Xembed support? Create a window with tabbed¹ in a tiling layout, open st in to it with "st -w <xid> -e true". If it is close to the monocle time, it is probably the other windows handling the resize event that is causing the slowdown not the layout choice.
To prove it to myself: I'm using river² and I can see a doubling-ish of startup time with foot³, iff I allow windows from heavier apps to handle the resize event immediately. If the time was a little longer(or more common) I'd be tempted to wrap the spawn along the lines of "kill -STOP <other_clients_in_tag>; <spawn & hold for map>; kill -CONT <other_clients_in_tag>" to delay the resize events until my new window was ready. That way the frames still resize, but their content resize is delayed.
To prove it to myself: I'm using river² and I can see a doubling-ish of startup time with foot³, iff I allow windows from heavier apps to handle the resize event immediately. If the time was a little longer(or more common) I'd be tempted to wrap the spawn along the lines of "kill -STOP <other_clients_in_tag>; <spawn & hold for map>; kill -CONT <other_clients_in_tag>" to delay the resize events until my new window was ready. That way the frames still resize, but their content resize is delayed.
¹ https://tools.suckless.org/tabbed/
² https://codeberg.org/river/river
³ https://codeberg.org/dnkl/foot