It is not the search engine software or mysql server, it is your server resources, if you are hosting your site on shared hosting account, then your hosting company places thousands on websites on the same server and they limit the server resources for each website and for each connection etc, so normaly they limit the RAM for each website in the range 10MB and normally these servers may be less than 1GB speed, so for the first few tens of thousands of links, these resources can be used by the mysql server to load all the tables to the memory or cash so it does the job as fast as expected, when you as said reach 100,000 sites added, it started to slow down when submitting, this is normal, with this shared server and the small amount or RAM, it needs a more time to sort and index all the keywords tables and the sites tables and their indexes.

If you want a pwerfull search engine with hunderds of thusands or listings, then get a powerfull dedicated server with say 3GB ram and set the Swap (Cash) partition large enough and optiomize your mysql server settings normally the file my.ini check , optiomize your webserver normally Apache, etc , etc.

Also check your search engine admin settings, you may be selected to index the description of the sites, if you did this realy optiomizes the search results but it takes a lot of time to include all the description sepcially if they are lengthy in the keywords tables.

Mewsoft Support
www.mewsoft.com