PostgreSQL Query Performance: Sorting Array Length vs. CTE

I'm having a significant performance difference between two PostgreSQL queries that I'm trying to understand and optimize. I have a table with around 2TB of data, and both queries use the sequential scan. The first query executes quickly, while the second one takes more than 2 hours to complete. Here are the queries and some details:

1st Query

WITH cte_mytable AS (   SELECT *, coalesce(array_length(tags, 1), 0) as size FROM my_table limit 100)SELECT  user_id, ...some other columnsFROM  cte_mytableWHERE  cte_mytable.user_id=user_idORDER BY  size desc;

Explain log:

|QUERY PLAN                                                                                         ||---------------------------------------------------------------------------------------------------||Sort  (cost=13.02..13.27 rows=100 width=593)                                                       ||  Sort Key: cte_my_table.size DESC                                                                     ||  ->  Subquery Scan on cte_my_table  (cost=0.00..9.70 rows=100 width=593)                              ||        Filter: ((cte_my_table.user_id)::text IS NOT NULL)                                             ||        ->  Limit  (cost=0.00..8.70 rows=100 width=593)                                            ||              ->  Seq Scan on my_table  (cost=0.00..197608426.93 rows=2272637194 width=593)|

2nd Query

SELECT *FROM my_tableORDER BY array_length(tags, 1) descLIMIT 100;

Explain log

|QUERY PLAN                                                                                                 ||-----------------------------------------------------------------------------------------------------------||Limit  (cost=217229180.49..217229192.16 rows=100 width=593)                                                ||  ->  Gather Merge  (cost=217229180.49..438195445.87 rows=1893864328 width=593)                            ||        Workers Planned: 2                                                                                 ||        ->  Sort  (cost=217228180.47..219595510.88 rows=946932164 width=593)                               ||              Sort Key: (array_length(tags, 1)) DESC                                                       ||              ->  Parallel Seq Scan on my_table  (cost=0.00..181037114.05 rows=946932164 width=593)|

Version

PostgreSQL 14.6

Can someone help me understand why the second query is so slow and the first query is faster?