Hellou.
This is initial bid abd time frame, couse as i emplain below, there is too few info.
Im php / mysql / html programmer with over 12yrs of experience, specialised with osCommerce stores.
During that time i was preparing wide sort of mods and integrations, and for many of them special actions were needed to assure high performance and stability.
If you are using standard shared hosting then we have a few options less to use, but it still can be a star performance boost.
Ive been working on diffrent environments, starting from shared hostings and customer made mods, that were blocking whole server (sometinmes 1 query), ending on hight volume multiserver stores. As an example of the last one i can give music store, that is using 2 diffrent high power servers that are supporting DB with over 1.500.000 products... all on osCommerce engine.
For some pages, diffrence between load time / query time / count of queries before and after optimization was counted in dozens of times less :)
If needed - and possible with your hosting - we could even install additional search server to speed up some things.
For now, and as few info as you gave its hard to say anything for sure.
If you are intrested in further talks we could do some pre-check before you grant the job.
The most helpfull would be some slow logs of mysql if you have some. Based on that we could see how bid problem it can be and if its mainly sql or script based.
All the best in The New 2015 Year,
Trygon eXtreme