Try to use my solution but use T1 for the table with more rows and T2 with the table which has less rows - and index on the timestamps (or use them as primary keys) so that the > and <= comparisons can go straight to the records instead of having to scan.
Also the Window function may be able to optimize a bit more as you always know that you reuse the timestamp rom just one record before.
~~~
SQLite\Examples>sqlite3 search.sqlite
SQLite version 3.35.5 2021-04-19 18:32:05
Enter ".help" for usage hints.
sqlite> .mode box
sqlite> select * from NearestMatch where tsPrev notNull;
┌───┬────────┬─────┬──────┬─────┐
│ a │ tsPrev │ ts │ ts:1 │ b │
├───┼────────┼─────┼──────┼─────┤
│ B │ 1.0 │ 1.2 │ │ │
│ C │ 1.2 │ 1.8 │ │ │
│ D │ 1.8 │ 2.5 │ 2.0 │ 50 │
│ D │ 1.8 │ 2.5 │ 2.5 │ 100 │
│ E │ 2.5 │ 3.6 │ │ │
│ F │ 3.6 │ 4.5 │ 4.5 │ 200 │
│ G │ 4.5 │ 4.6 │ │ │
│ H │ 4.6 │ 9.8 │ 8.5 │ 300 │
└───┴────────┴─────┴──────┴─────┘
~~~
created by following example SQL statements:
~~~
drop table if exists T1;
create table if not exists T1 (
ts real not null primary key,
a char not null
);
drop table if exists T2;
create table if not exists T2 (
ts real not null primary key,
b integer not null
);
replace into T1(ts, a)
values (1.0, 'A'), (1.2, 'B'), (1.8, 'C'), (2.5, 'D')
, (3.6, 'E'), (4.5, 'F'), (4.6, 'G'), (9.8, 'H');
replace into T2(ts, b)
values (2.0, 50), (2.5, 100), (4.5, 200), (8.5, 300);
drop view if exists NearestMatch;
create view NearastMatch as
with timeFrames as (
select first_value(T1.ts) over win as tsPrev, *
from T1
WINDOW win AS (ORDER BY T1.ts ROWS BETWEEN 1 PRECEDING AND 1 PRECEDING)
)
select TF.a,TF.tsPrev,TF.ts, T2.*
from timeFrames TF
left outer join T2 on T2.ts > TF.tsPrev and T2.ts <= TF.ts;
select * from NearestMatch;
~~~
Here the windowing function last_value() or first_value() may be faster than lag() or lead() and the range is limited just to the one and only previous record.