[@KatherinWinter]
It takes prolonged physical contact to locate and extract the specific memory, longer for formulating replacements. The older and more important a memory is the longer it takes and the more difficult it is to extract. Core memories, the ones that like you said cause a life-long fear, can take hours to extract as a Djinn would need to search through their memory to find it, sever EVERY connection it has to other memories, and then reconnect the links between memories that removing it created.

During the period where a memory extraction and replacement is being performed, they must maintain their natural Djinn form (at least one full limb) which is highly conspicuous at best, especially since any clothes they are wearing in their human form are part of the shell and will dissapear along with the human limb while they are using their natural reptilian arm for the process.

To perform a proper memory extraction (which causes temporary paralysis) they must place their hand on a person's face, direct skin to skin contact. While they can extract memories with any direct contact, shaking hands would not cause the paralytic effect and they'd only have a couple seconds to work before the other person realizes they're shaking hands with a lizard wearing a human suit, meaning at best they could skim off the memories of the last few minutes.

If the process is interrupted before they fully extract a memory (even if they're only take a few more seconds), it will 'bounce back' into the owner, effectively reversing any changes made, so they'd have to start over to try and extract the same memory.

It is harder to extract the memories of more intelligent or strong willed people, or those with psychic powers, as they are naturally resistant to mental magic.