If you are using postgresql-libpq's escapeStringConn,  or a library that uses it (e.g.  postgresql-simple,  or persistent-postgresql),  I do recommend upgrading.   You may or may not see a performance improvement,  depending on your particular use case,   but if you do it can be quite substantial.

It's not entirely clear to me what the root cause really is,  but it certainly appears as though it's related to the (direct) use of mallocBytes,   which was replaced with (indirect) calls to mallocForeignPtrBytes / mallocPlainForeignPtrBytes (through the bytestring package).   In this case,  it resulted in an asymptotic improvement in time complexity of some algorithms.