MySQL Query with Inner Join to small table suddenly takes FOREVER

Below are 2 queries.  They're exactly the same except for the inner join to the t_lob table (which has 14 rows in it and an index on account and skill).  The query with the join used to run in a matter of seconds before I upgraded to MySQL v5.6 (from 5.5).  Now it crawls.  

Why? What can I do to fix it?  

Here are the explain results:

Query with Join:
id,select_type,table,type,possible_keys,key,key_len,ref,rows,Extra
1,PRIMARY,<derived2>,ALL,NULL,NULL,NULL,NULL,6066,"Using temporary; Using filesort"
2,DERIVED,l,ref,"Account,Skill",Account,11,const,6,"Using index condition; Using where"
2,DERIVED,t,ref,"StartDate,Skill,Account",Skill,103,att.l.Skill,1011,"Using where"


Query with Join Commented out:
id,select_type,table,type,possible_keys,key,key_len,ref,rows,Extra
1,PRIMARY,<derived2>,ALL,NULL,NULL,NULL,NULL,445155,"Using temporary; Using filesort"
2,DERIVED,t,range,"StartDate,Account",StartDate,4,NULL,593540,"Using index condition; Using where"



.....So, it seems to me that the inclusion of the inner join causes fewer rows to be evaluated.  However, as I've explained it actually takes longer. Not even sure if it will complete.  I've let it go for about 30 min before killing it.  The version without the join takes 10.8 seconds

To clarify.  t_transcripts has about 10M rows in it.  t_lob has 14.


select distinct
#lob,
URL_Cleaned, count(URL_Cleaned) as URL_Count, sum(convcounter) as Conversions

from (select  
t.account,  
#l.lob,
#l.sortseq,
t.sessionid,
t.skill,  
t.startdate,  
t.chat_button_name,
t.chatreferer as URL,
substring_index(substring_index(substring_index(t.chatreferer, '://',-1),'#', 1), '?',1) as URL_Cleaned,
if(isnull(conversion), 0, 1) as convcounter

from  
t_transcripts t  

#inner join t_lob l
#on t.account = l.account
#and t.skill = l.skill  

where  
t.account = '12345678'
and t.startdate between '2014-02-12' and '2014-02-28'
)q  

group by
#lob,
url_cleaned



-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SLOW QUERY (inner join is not commented out):




select distinct
#lob,
URL_Cleaned, count(URL_Cleaned) as URL_Count, sum(convcounter) as Conversions

from (select  
t.account,  
#l.lob,
#l.sortseq,
t.sessionid,
t.skill,  
t.startdate,  
t.chat_button_name,
t.chatreferer as URL,
substring_index(substring_index(substring_index(t.chatreferer, '://',-1),'#', 1), '?',1) as URL_Cleaned,
if(isnull(conversion), 0, 1) as convcounter

from  
t_transcripts t  

inner join t_lob l
on t.account = l.account
and t.skill = l.skill  

where  
t.account = '12345678'
and t.startdate between '2014-02-12' and '2014-02-28'
)q  

group by
#lob,
url_cleaned