Content-Modification System with Client-Side Advertisement Caching

In one aspect, a method includes (i) identifying a plurality of upcoming content-modification opportunities for a content-presentation device, each respective content-modification opportunity corresponding to a respective underlying advertisement that is available for replacement by a respective supplemental advertisement; (ii) using the identified upcoming content-modification opportunities as a basis for determining expected values for a plurality of supplemental advertisements; (iii) selecting a subset of supplemental advertisements from among the plurality of supplemental advertisements based on the subset having expected values above a threshold value; (iv) in advance of the upcoming content-modification opportunities, sending the subset of supplemental advertisements to be locally stored at the content-presentation device; and (v) upon occurrence of one of the content-modification opportunities, causing the content-presentation device to replace one of the respective underlying advertisements with one of the locally stored supplemental advertisements.