IC 6.1 has this problem, (slow display and need more memory)
The other reason, your display data maybe too big.
Increase your system memory size might help.
The other way, try "top view" then "in-depth view",
Ctrl-f ---> move ---> Shft f
or
bind-key "e" --> choose stop-level 0 (equivlence to Ctrl f) --> OK