Hi,
$rpacor is the amount of memory that is used to store excited state vectors while $ricore is used to store RI-J integrals during SCF or CPKS iterations (comparable to saving integral files to disk in Hartree-Fock, just that RI-J integrals can be recomputed much faster than any current I/O can deliver data, so they can be kept in RAM instead).
Using less $ricore will affect the speed of the calculation not dramatically, say 10-50%. But I have no hard numbers for that since it depends on the size of the input (but you can check it yourself by setting $ricore to 0 and to as much as your computer has minus 20%). So if your machine has 1GB, $ricore of 700 to 800 should be o.k. The problem is that $ricore gives just the amount of memory used for RI, all other matrices like Fock-matrix, density, DIIS matrices, etc. need additional memory! The larger your input, the larger the amount of memory that is needed for the non-RI part. Set $ricore to about 70-80% of your total memory, run ridft or escf or whatever and check the memory used with 'top'.
$rpacor (like $maxcor for MP2/CC2) on the other hand has much more influence on the speed of your TDDFT calculations. If you have enough memory, all excited states can be computed in one sweep. If you give less than needed, escf/egrad will loop over the excited states, doing as many of them in one loop as fit in the memory. So increasing $rpacor for a calculation with a couple of excited states can give a speed up of more than 2, 3, etc. - depends on your input and memory settings.
To be brief:
If you do TDDFT, $rpacor is much more important than $ricore !
Best,
Uwe
