My guess is the answer is NO. You won't make this any faster.
StringBuilder has extra room for extra text, which is appended at the end of the already used space. When it actually runs out of space, it allocates a big chunk for further filling. A
String is immutable and does not have extra room to add new text "at the end".
Perhaps what you should do is not modify your original row, and have instead a secondary related table where you store every string associated with
myKey, together win an
string_order column (could be an auto_increment, or a current timestamp) and, when needed, retrieve everything together with a
GROUP_CONCAT of them. Your related table would just work as a kind of log, where you would be inserting the new "events" in order, a piece at a time.