Generally, yes. A little less so if you have spare but still important.
The main reason is, as you said, cache. The more unused RAM you have, the longer caches will be kept. It includes disk cache, browser cache, etc... But it can also make the CPU cache more efficient. L2 cache is about 10 times faster than RAM, and the less RAM you are using, the more cache will be used. L1 cache is even faster but here, access patterns matters as much as raw size, if not more.
The main reason is, as you said, cache. The more unused RAM you have, the longer caches will be kept. It includes disk cache, browser cache, etc... But it can also make the CPU cache more efficient. L2 cache is about 10 times faster than RAM, and the less RAM you are using, the more cache will be used. L1 cache is even faster but here, access patterns matters as much as raw size, if not more.