how to optimize data aggregation based on value range conditions applied on two columns

I have a particles table in PostgreSQL 10.19 that looks like this:

CREATE TABLE particles (
   particle_diameter REAL, 
   particle_speed REAL
);


INSERT INTO particles VALUES
   (0.35, 0.74), 
   (0.57, 2.63), 
   (0.27, 1.05), 
   (0.65, 2.33);

What I want is to aggregate my data by diameter range (i.e. each particles that have a diameter between for example 0 and 0.2 mm, 0.2 and 0.4 mm, etc.) and speed range. I want to generate a serie of diameter range, and a serie of speed range and finally count the number of particles for each duo diameter range and speed range.

So far I managed to reach the desired result using this query:

WITH speed_series AS (
   SELECT generate_series(-1, 19.8, 0.2) AS speed_from
), speed_range AS (
   SELECT speed_from, (speed_from + 0.2) AS speed_to FROM speed_series
), diameter_series AS (
   SELECT generate_series(0, 9.8, 0.2) AS diameter_from
), diameter_range AS (
   SELECT 
      diameter_from, (diameter_from + 0.2) AS diameter_to, speed_from, speed_to
   FROM diameter_series, speed_range
)
SELECT
   diameter_from,
   diameter_to,
   speed_from,
   speed_to,
   (SELECT 
       COUNT(particle_diameter) 
    FROM particles
    WHERE particle_diameter BETWEEN diameter_from AND diameter_to
    AND particle_speed BETWEEN speed_from AND speed_to
   )
FROM diameter_range;

You can explore it on this :
db<>fiddle

On a relatively small dataset (~30k records) this query took more than a minute to execute. So my question is:

Is there a way to rewrite this query to be more efficient and less time consuming?

I would try width_bucket() or floor() here:

with b as (
  select width_bucket(particle_diameter, 0, 10, 50) pd,
         width_bucket(particle_speed, -1, 20, 105) ps
  from particles)
select pd * .2 - .2 diam_from, pd * .2 diam_to, 
       ps * .2 - 1.2 speed_from, ps * .2 - 1 speed_to,  
       count(1) cnt 
from b group by pd, ps

Of course you can join with generated series if you need these zero-rows:

with b as (
  select width_bucket(particle_diameter, 0, 10, 50) pd,
         width_bucket(particle_speed, -1, 20, 105) ps
  from particles)
select d diam_from, d+.2 diam_to, s speed_from, s+.2 speed_to, coalesce(t.cnt, 0) cnt 
from (select generate_series(0, 9.8, 0.2)) as dm(d)
cross join (select generate_series(-1, 19.8, 0.2)) AS sp(s)
left join (
    select pd * .2 - .2 diam_from, ps * .2 - 1.2 speed_from, count(1) cnt 
    from b group by pd, ps) t
  on dm.d = t.diam_from and sp.s = t.speed_from

dbfiddle demo