Step 1: Check the Reason first.
Ultrafiltration is indeed carried out under pressure through a special membrane with extremely fine pores, and it is known to be a slow process because the membrane pores are very small and resist flow. So Reason (R) is a true statement on its own.
Step 2: Now check the Assertion.
Colloidal particles are typically in the size range of $1$ to $1000$ nm. Ordinary filter paper has pores far larger than this range, so colloidal particles pass straight through it without being trapped. Assertion (A) is also true.
Step 3: Ask whether R actually explains A.
The reason colloids pass through ordinary filter paper is purely a matter of particle size versus pore size of that paper, nothing to do with ultrafiltration at all. Ultrafiltration is a completely different, separate technique used specifically because ordinary filtration fails for colloids.
Step 4: Spot the mismatch.
Saying "ultrafiltration is slow" tells us nothing about why colloids slip through plain filter paper. A correct explanation for (A) would talk about pore size of the filter paper, not the speed of an unrelated technique.
Step 5: Conclusion.
Both statements are individually correct, but (R) is not the reason behind (A).
Step 6: Final answer.
\[ \boxed{\text{Both (A) and (R) are correct, but (R) is not the correct explanation of (A)}} \]