Yes, the FAQ says it is completely automatic. However, is there a way to switch it off?

Forgive me for the very imprecise question. The situation is as follows: we have some monster expressions where we need to substitute a few thousands of symbol's occurrences with something else. We did that by implementing our own recursive tree traversal. However, we estimate that more than 80% of the time is spent in attempted simplifications by GiNaC (which we know are deemed to fail). Is there any way to implement this substitution process so as to avoid this big overhead? Thanks in advance

Roberto