Bitcoin determines the best chain by choosing the one with the most work. As "work" is a bit nebulous, it is specifically determined using a metric called "chainwork". Chainwork is just the expected number of hashes required to produce all of the blocks in the chain. It is calculated by computing the expected number of hashes required to produce the block, and that is added to the chainwork of the previous block. As such, chainwork can only increase.
Bitcoin Core's assumevalid mechanism also includes a minimum chainwork value. This is set to be the chainwork of the assumevalid block. Any chain that Bitcoin Core accepts will have to have a chainwork greater than this value.
With the minimum chainwork, Bitcoin Core can filter out low difficulty chains. As it does headers first sync, it will download the block headers for the low work chain, see that its chainwork is less than the minimum chainwork, and not download the blocks for that chain. It will additionally disconnect the peer that advertised that chain to it.