I think Thunderbird does this search internally, not via IMAP. You can test this by talking IMAP protocol directly:
telnet localhost 143 a login user pass b select inbox c search text hello
Yes, you definitely want to check things are being accelerated by issuing direct IMAP commands via telnet. Many clients try to "help" by performing local searches, which will only obfuscate things for you. Even with 150K+ messages, it shouldn't take fts_solr more than 20ms or so to give you results.
I too was bitten by the configuration issue. The wiki/docs suggest that you only need to put the fts fts_solr plugin spec into imap "section", which never worked for me, unlike fts_squat which did). Putting it into the "global" plugin list made it all work for me.
You can check your solr index data directory too. A freshly installed solr index occupies almost no space, but that grows QUICKLY once it's indexed anything.
=R=