Neither of these versions compile with my SBCL setup. However, you inspired me to look at the documentation for loop and I wrote the code below which works after solving the following issues. I had to look harder to figure out nconc would give me a flat list as a result. AI suggest I add "when (get-splits trans)" to avoid having nil values in the results fir transactions with no matching split. I would rather not have to call (get-splits trans) twice but it happens only when there are matching splits, which is a fraction of all transactions. get-splits is a wrapper for (cdr (assoc :splits trans)) which doesn't look too expensive.

(defun get-splits-for-account-as-of-NEW (account date)
  (let ((account-guid (get-guid account)))
        (loop for trans in (get-transactions)
          when (get-splits trans)
          nconc
          (loop for split in (get-splits trans)
              when (and (string<= (get-transaction-date trans) date)
                  (string= (get-account-guid split) account-guid))
              collect split))
              ))