Mysql UNION ALL query - Too slow using longtext

I have this query

select t.field1 from (
select t1.field1, t1.field2 from db1.table t1
UNION ALL 
select t2.field1, t2.field2 from db2.table t2
UNION ALL 
select t3.field1, t3.field2 from db3.table t3
) as t

field1 is a normal varchar(255) field, but field2 is a longtext field with > 3000 characters of data. If I am using this query > 20 sec, but if I am using without field2, this query < 2 sec. Theese tables have 100.000 rows but it will expand. What is the solution of this problem?