Table very slow

HI, i have one table as 15 millions of rows of all 2013. I all days delete the las 2 months and return to reload them again. Today is table is very slow, one select count takes more that 2 minutes, when it would normally to a few seconds. I migrate this information to a new table and this table if works fine and is very fast. What i need to do for to fix this and how i to fix this table slow?

Thansk

Edgar M