How to Analyze database performance

Question

I have a 4 TB data base.

The use is more as a data warehouse.

Using the PostgreSQL DB +Django.

structure looks like Instagram:

class Influencer(models.Model):
    followers = models.ManyToManyField(
        "instagram_data.Follower", verbose_name=_("Followers"))

class Follower(models.Model):
    is_private = models.BooleanField(_("is_private"), default=False, db_index=True)
    is_verified = models.BooleanField(_("is_verified"), default=False, db_index=True)
    is_business = models.BooleanField(_("is_business"), default=False, db_index=True)
    business_category = models.CharField(
        _("business_category"), max_length=254, null=True, blank=True, db_index=True)
    media_count = models.BigIntegerField(_("media_count"), default=0, db_index=True)
    follower_count = models.BigIntegerField(_("follower_count"), default=0, db_index=True)
    following_count = models.BigIntegerField(_("following_count"), default=0, db_index=True)
    gender = models.CharField(
        _("gender"), choices=Gender.choices, null=True,
        blank=True, max_length=50, db_index=True)
    language = models.ManyToManyField(
        "instagram_data.Language", verbose_name=_("language_code_name"), blank=True)

Where each Followers has many "filters".

I first choose X influences, later choose the followers Filter (is_private and so ..)

It does take some time to get results (like 30 sec for each 100K users).

Most filters are Indexed as you can see.

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What can I do to speed up things ?

How can I tell if the bottleneck is the Disk i/o bounds and what speed should the disk be ?