civicrm/member/add&reset=1&action=add&context=standalone has bad query -it takes a loong time when there are a significant amount of records

When adding a member in our DB with ~20,000 contacts, ~70,000 contribs, ~15,000 members on xavier's superserver, generating civicrm/member/add&reset=1&action=add&context=standalone took about a minute to generate.

using define( 'CIVICRM_DEBUG_LOG_QUERY', 'backtrace' ) worked out that it was looking for contributions when it didn't need to so made two adjustments to

1) only ran the find membership contributions query if a membership_id is present

2) don't both looking for membership contributions if you are adding a new member

checked on pages that do need to find associated contributions and it seemed to find them OK