SQL内部联接性能问题

我安装了WordPress，正在制作一个自定义插件。我的插件存储了很多usermeta。我使用内部联接来组合来自"users"answers"usermeta"表的数据，此时我将结果吐回我的PHP脚本。以下是我的查询：

select
# Users table stuff
users.ID,
users.user_email,
users.display_name,
# Meta stuff
first_name.meta_value as first_name,
last_name.meta_value as last_name,
phone_number.meta_value as phone_number,
country.meta_value as country,
years_of_experience.meta_value as years_of_experience,
highest_degree_obtained.meta_value as highest_degree_obtained,
availability_for_work.meta_value as availability_for_work,
english_proficiency.meta_value as english_proficiency,
disciplines.meta_value as disciplines,
profile_picture.meta_value as profile_picture,
resume.meta_value as resume,
description.meta_value as description,
hourly_rate.meta_value as hourly_rate,
satisfaction_rating.meta_value as satisfaction_rating,
invited_projects.meta_value as invited_projects,
completed_project_count.meta_value as completed_project_count
# The table we're selecting from
from tsd_retro_users as users
# Join in our usermeta table for each individual meta value
inner join tsd_retro_usermeta first_name on users.ID = first_name.user_id
inner join tsd_retro_usermeta last_name on users.ID = last_name.user_id
inner join tsd_retro_usermeta phone_number on users.ID = phone_number.user_id
inner join tsd_retro_usermeta country on users.ID = country.user_id
inner join tsd_retro_usermeta years_of_experience on users.ID = years_of_experience.user_id
inner join tsd_retro_usermeta highest_degree_obtained on users.ID = highest_degree_obtained.user_id
inner join tsd_retro_usermeta availability_for_work on users.ID = availability_for_work.user_id
inner join tsd_retro_usermeta english_proficiency on users.ID = english_proficiency.user_id
inner join tsd_retro_usermeta disciplines on users.ID = disciplines.user_id
inner join tsd_retro_usermeta profile_picture on users.ID = profile_picture.user_id
inner join tsd_retro_usermeta resume on users.ID = resume.user_id
inner join tsd_retro_usermeta description on users.ID = description.user_id
inner join tsd_retro_usermeta hourly_rate on users.ID = hourly_rate.user_id
inner join tsd_retro_usermeta satisfaction_rating on users.ID = satisfaction_rating.user_id
inner join tsd_retro_usermeta invited_projects on users.ID = invited_projects.user_id
inner join tsd_retro_usermeta completed_project_count on users.ID = completed_project_count.user_id

# Define our select stipulations
where 
(users.ID = 20)
and
(first_name.meta_key = 'first_name')
and
(last_name.meta_key = 'last_name')
and
(phone_number.meta_key = 'phone_number')
and
(country.meta_key = 'country')
and
(years_of_experience.meta_key = 'years_of_experience')
and
(highest_degree_obtained.meta_key = 'highest_degree_obtained')
and
(availability_for_work.meta_key = 'availability_for_work')
and
(english_proficiency.meta_key = 'english_proficiency')
and
(disciplines.meta_key = 'disciplines')
and
(profile_picture.meta_key = 'profile_picture')
and
(resume.meta_key = 'resume')
and
(description.meta_key = 'description')
and
(hourly_rate.meta_key = 'hourly_rate')
and
(satisfaction_rating.meta_key = 'satisfaction_rating')
and
(invited_projects.meta_key = 'invited_projects')
and
(completed_project_count.meta_key = 'completed_project_count')

正如您所看到的，对于我试图从数据库中获得的每个值，我都会对usermeta表进行内部联接。这一切似乎都很好，但在进行了一定数量的内部联接后，查询似乎慢到爬行。现在，上面的查询平均需要大约一秒钟的时间，而我的用户表中只有大约20个用户。

我的问题是：内部联接的性能后果是什么？有没有更好的方法来运行上述查询并获得相同的结果？