Please let me know what information you seek to improve the question, rather than just downvoting.

I have a function that looks like this:

DELIMITER $$
DROP FUNCTION IF EXISTS f_splitadjprice;
CREATE FUNCTION f_splitadjprice (id CHAR(8), startdate DATE)
RETURNS FLOAT 

BEGIN

DECLARE splitfactor FLOAT;
DECLARE splitadjprice FLOAT;

SELECT f_splitfactor(id, startdate) INTO splitfactor;

SELECT (f.p_price FROM fp_v2_fp_basic_prices as f WHERE f.fsym_id = id AND 
f.p_date = startdate) * splitfactor INTO splitadjprice;

RETURN splitadjprice;

END$$
DELIMITER ;

The function for splitfactor is:

DELIMITER $$
DROP FUNCTION IF EXISTS f_splitfactor;
CREATE FUNCTION f_splitfactor (id CHAR(8), startdate DATE)
RETURNS FLOAT

BEGIN

DECLARE splitfactor FLOAT;

SELECT IFNULL(EXP(SUM(LOG(f.p_split_factor))),1) INTO splitfactor
FROM fp_v2_fp_basic_splits AS f
WHERE f.fsym_id = id AND f.p_split_date > startdate AND f.p_split_date < 
NOW();

RETURN splitfactor; 

END$$
DELIMiTER ;

The function f_splitadjprice runs extremely slow. About 14 seconds PR row. I have tried to run the individual pieces of the function by themselves. That is, the function call f_splitfactor and SELECT (f.p_price FROM fp_v2_fp_basic_prices as f WHERE f.fsym_id = id AND
f.p_date = startdate). When running these two by themselves outside of the function they take 0,001 seconds to run. So the whole problem is that as soon as I want to do in combination through the nested function it takes 100.000 times longer?

The solution is to not call tables within functions. In general that seems to be bad practice and it is nevertheless extremely slow. One should instead try to get rid of the function and perform the function directly in the query.