SELECT query waiting on DataFileWrite in Postgres - postgresql

Question

I have a partitioned table on which I run both queries and INSERTs simultaneously from multiple sessions. Database is Postgres 14.1.
Running select wait_event_type, wait_event, state, query from pg_stat_activity often reports my queries (plain SELECT on an indexed column) are waiting on DataFileWrite wait_event, type IO.
I don't understand, why would a query wait on file write?
Limit (cost=210.00..210.00 rows=1 width=14) (actual time=3.279..3.281 rows=0 loops=1)
Buffers: shared hit=27 read=7
I/O Timings: read=3.044
-> Sort (cost=210.00..210.11 rows=43 width=14) (actual time=3.278..3.280 rows=0 loops=1)
Sort Key: mldata.sinceepoch DESC
Sort Method: quicksort Memory: 25kB
Buffers: shared hit=27 read=7
I/O Timings: read=3.044
-> Append (cost=0.56..209.79 rows=43 width=14) (actual time=3.274..3.276 rows=0 loops=1)
Buffers: shared hit=27 read=7
I/O Timings: read=3.044
-> Index Scan using mldata_313_destination_idx on mldata_313 mldata_1 (cost=0.56..20.67 rows=4 width=14) (actual time=0.056..0.056 rows=0 loops=1)
Index Cond: (destination = 154328765)
Filter: (day = ANY ('{313,314,315,316,317,318,319,320}'::integer[]))
Buffers: shared hit=6 read=1
I/O Timings: read=0.010
....... a few more of IndexScan .........
Planning:
Buffers: shared hit=1340 read=136 written=11
I/O Timings: read=0.910 **write=0.138**
Planning Time: 3.920 ms
Execution Time: 3.338 ms